It looks like the kid is leaning towards the household he sees as buttering his bread.  He invited his dad.. and probably didn't realize how hurt his dad would feel that he might feel excluded.

I think the message to the bio mom was a mistake.. she probably lapped that up.

I think a discussion with his son as to what his thought process was.. and how it hurt him as a father to feel excluded would have been the way to go.  I wouldn't have done it in writing... face to face.. man to man.. so to speak.

I've been through a senior night, and some kids had 5 people walk with them. Included grandparents and uncles. Little Damian knew what he was doing.

One thing i have seen with my SO's son, is that these dysfunctional relationships blow hot and cold. They are just messy and up and down. It can't be good for the kids in the long run. Their character development will suffer. I kept SS19 at my house overnight 50/50 for almost a year. Taught him to drive. Brought him where he needed to go. But the judge ordered therapy for him and BM so she could keep her child support, even though he hasn't stayed with her in 6 years. She's "in" and i'm chopped liver. Whatevs. I have my own kids but it does hurt. 

My DH went thru that at Homecoming with OSD.  Not even stepdad, but BM's long term boyfriend.   
 

The only question SS needs to be asked is why his mother thought that was appropriate?  
 
Good enough DNA to make a kid with but trash for everything else?  That's the biggest line of BS.  I feel your pain.   I hope he doesn't chase SS after that stunt.  He should get nothing.  There's no taking the high road on this one.  The smirk? He know exactly what he did.  No senior gift, trip, car, whatever he had planned for SS , forget it.  He needs logical consequences for a public betrayal on this level.  
 

This kind of wound is deep.  Very deep.   

BM1 had 5 or 6 guys play stepdaddy to the children. Thats without counting the flings/one night stands who would overstay.

OSD was a cheerleader and frequently invited these random "dads" to watch her perform. One of the "dads" took her and her mother out for her bday which pissed my husband off 

She always kept him out of these things because BM1 would threaten to cancel bdays or take her out of cheer if she invited her dad to any of these things. When she graduated from some accelerated high school program, my husband encouraged her to walk on stage and said he was very excited to be there for that moment (first born graduating high school) but she told him she thought it was stupid and had no interest in doing

Fast forward months later and she walks on stage with all of BM1s family there and takes pics that she puts on social media. Her excuse was "mom told me not to tell you and that she wouldnt come if you were there"

My husband cried and stuff like that but honestly I dont care that much. I felt a bit bad for him but its not my business. Not my monkey, not my circus. Thats just how it is with these dysfunctional parents

Since then no one has graduated or even managed to finish high school so we are good on that front. I doubt YSS15 will finish either. Probably another high school drop out lol

What a weird family and weird dynamics

My suggestion to you is to stay out of it and maybe give emotional support to your spouse if you can but these things cannot be changed and clearly his son wanted the BM and the stepdad to walk him or else he would have responded to his fathers text the next day and explained that there was a mix up but ignoring it means this was all pre planned and thought out by both his son and his BM

It's like some sick game of limbo - "how low can you go?"

Broke up the marriage.  I can see not having a friendly relationship.  So at this type of events someone is going to be left out.  And other events to follow. You DH must assert he lost and cut off SS.  NO college money no car money.  No making a wedding. 
 

Was 50/50 ever awarded by a court?  If not, DH needs to go get his kid and not allow any interface between SS and his whore of a mother except per the COd visitation schedule.

She fails to return the SKid per gthe visitation order, daddy nails her in court with a contempt motion.

Time for this kid to get the full frontal facts about his whore mother and her cheat partner sperm donor on her spawn #2.

Every sordid fact and detail. Over, and over, and over again.

This Skid needs the truth and do understand clearly that mommy abandoned him, lies to him, and that he (SS) and his BIoDad are the victims of whore mommy's and her cheat buddy dick donor's crap.

Grrrrrr.

I for damned sure would do everything in my power legally, financially, and socially to destroy that skank whore and her adultery partner.  My kid would know ever detail and would know that his entire life will require that he defend himself from the lies, toxicity, and maniplations of his skank whore of a mother. Not just while he is continuing to grow up, but his entire life as an adult.

Adultery is a trigger point for me.  My XW was a serially adulterous whore.  The only positive out of that blessedly short marriage (2.5  years) is that I did not pollute my gene pool with her.  Had we had a child, the past nearly 35yrs of my life would have been significantly spent destroying her and protecting my child(ren) from her.   I an infuriated for your DH and how his kid has beein drinking at BM's adulterous whore tit and is wallowing in the shallow and polluted end of his gene pool.

We did defend my SS, our marriage, and our blended family buy applying brutally aggressive overwhelming consequences for the serial statutory rapist Spermidiot, the ranty harpy of a SpermGrandHag, and the rest of the shit puddle gene pool SpermClan.  Our son has thrived as an adult and regularly comments on how grateful he is that his mom and I raised him with standards and presented him with the facts regarding his blended family situation.  Kids need the facts. To defend themselves from the crap perpetrated against them my the toxic end of their gene pool. As children, and as adults. People like this never stop their toxic crap.  Kids will never get away from it until the POS parent dies. Which can't happen soon enough IMHO.

Even my very enlighted Skid struggles with baggage from his shit puddle of a SpermClan gene pool.  He has been increasingly aware of the facts since before he hit double digit ages and aged out from under the CO more almost 14 years ago.

Your DH involvement in SS life .  His invoked was he?  did he see his son every week , Did then do father son things.  It seems like SD is more of a parent then DH.  Does DH pay a fair amount of CS, or is SF Picking up the majors amount of money for SS care?  Or is SF is paying the major money for SS, and doing father son things with SS. Maybe SS feels SF is more of a father.

I really don't understand [PSTD]. BF paying $200 a month and think he's supporting the ex household. Where everybody else is picking up his responsibility.

And last of all, today with all the types of relationships, GP raising the GK,  sisters, brothers, raising kids. BM and her SO, BF and his SO. Schools can't get involved with all of this.  So it's normal anyone who help raised this kid walks with him .  It takes a village 

Just stood up, and walked to join my kid.  BM and her adulterous breeder fuck buddy could fuck off.  That would have been the point where I planted my stake in the ground and made clarity for my kid and ripping the throat out of BM the rest of my life entertaining hobby.

I'd suggest reading justmakingthebest's blog.  Her DH has also been through a very public discard by his son.  It might give you some ideas or at least reasure you that you aren't alone.

It seems that perhaps the time where his mom was absent made your SS crave her presence.. acceptance more.. it's not really that uncommon for people to pine after things that are out of reach.. and then she came back.. he likely has some underlying security that he will lose her if he doesn't play by her rules.

So.. yeah.. 18.. but probably a 18 yo who has been raised in a difficult way by two people that are not each other's biggest fan.

It sounds like the message is "be nice to my mom..or I can't have a relationship with you."

I am wondering.. what kind of relationship did your husband and his son have prior to BM coming back?  did his son want to be with mom because he and his dad were already butting heads?  could some of that have been because of your husband's feelings towards his ex?  and maybe the son now feels protective of mom?

Do your husband and his EX get along in public (previously).. has your SO sent her messages that could be taken as negative.. has he been dismissive of the mom to the boy in the past?  In other words.. if you honestly and objectively look at this.. do you think that your husband at all feeds into the conflict with the ex? are there scenes in public.. does he talk badly about her to others.. or his son?

If you can say that your husband has been nothing but gracious about it all.. the message his son should get is this. and by message.. I mean talk.. don't put things in writing to be twisted.. dad talks to his son.. in person.

"Son,  I know you love your mother and I have no intention of disrespecting her, but I do expect the same from her.  Do we love each other and agree about everything? No, of course not.. if we did, there would have been no reason for us to separate.  You have to understand that in order for us to want to break up our family, we both knew that we did not work as a couple.  Since then, we have both taken turns at raising you, and I'm surprised and hurt that you think I would do anything intentional to hurt you and I did not realize that my relationship with you was conditional and I certainly didn't raise you to treat me like you did at the senior day event.  If that's how you intend to move forward in your life.. I would appreciate being left off the invitation list entirely.. and I hope you understand that my willingness to support things in your life after graduation will probably be reconsidered if you don't view us as having a relationship.  I hope I have misunderstood what happened.. but if I haven't.. my door is always open.. and I love you.. but if you don't want a relationship with me.. I cannot force one.