She's ridiculous and you and DH should continue to spend your custody time as you see fit.  

I think that your bm there has the makings of a highly theatrical martyr. If she wants to dedicate her life to her offspring, let her. I also think that she must be a very sad person inside ... I mean, what kind of a life does she have if all she ever does is run after her kids? I actually feel sorry for that poor, misguided woman.

I think your BM is taking it way too far.  I think one sport or activity per season is fair, not multiple ones.  And as kids get older, they're going to want to do more things with friends, but there also still needs to be family time.  Your BM should not be scheduling anything on your husbands parenting time, such as accepting get togethers with friends or things like that.  At the end of the day, the parents are the ones that run the house, not the child.  

She's wrong. But you know that.

How much longer until court?

One activity is enough.  My daughter is in marching band and as much as I love watching the halftime show, the running around is exhausting.  Just imagine how exhausting it is for the kids.  My daughter has to work hard to show up for marching practice and keep up her grades.  

Meh... not everyone raises their kids in a child centric household.  If she wants to turn herself inside out fine, but she can't dictate what your husband does on his time.

My feeling is that the care and safekeeping for basic needs of minor children is the first priority.  So, they are warm, clean, fed have a bed and clean underwear? (could add access to education)... you have met your primary obligation.  This oblligation is a higher priority than your spouse, your stepkids.. even your job theoretically.    But, that is BASIC need to survive here.. that isn't baseball games etc...

After the priority list of everyone's basic needs are met.... you move on to the wants.  Those wants need to be weighed and prioritized along with the wishes and preferences of everyone else in your family unit.  Sometimes one person and their want is a priority.. sometimes it's someone else.. sometimes the household good is more of a priority than one individuals wants.  

So, Within reason, I think it's good to support kid's activities..but not to the exclusion of everyone else's life.  

If he were prioritizing taking your kids to baseball practice and they always got to attend their games.. and he was making his kids miss out?  That would be kind of messed up..   We can't please everyone all the time, but we can try to be fair about it.  

But, just because BM wants to have no life doesn't mean that her EX has to do the same.  He can make reasonable efforts as he can to allow them to participate in things.  

Because, to an extent, I think a parent should support their kid's activities to a point.. and that might mean from time to time the parent isn't doing what THEY want to do, but it doesn't have to be all consuming.

I guess it's a balancing act.  Obviously, dad didn't want the kid to do too many activities so there wouldn't be so many potential conflicts with other activities on his time.  Even so, it certainly would be good to understand what your child's potential obligations might be in the future and if it can be avoided... not double book things.. even if it might be on his time.. even for activities he may not have given absolute approval for.   I mean, most parents look at their kid's sport schedules and try to plan around them as long as it isn't an undue burden on the rest of the family to do that.  

The rub here is that his EX signs him up for so much stuff.. he is almost always going to have an obligation on dad's time.  Now, sometimes it might work out that dad can take him to that thing.. but sometimes it won't work out and hence part of the reason dad doesn't want multiple activities going on at once.  He is willing to work around one... but not more activities.  I am assuming OP's kids also have their own things going on too.  So, for this game.. it sounds like there is a family activity planned for the same day.. a direct conflict.  It's in dad's court whether he allows him to play.. or not.  Apparently missing games has not been an issue in the past but yeah.. the kid will likely be unhappy to miss it.  It puts dad in a pretty tough situation.. he can allow him to play.. but that means possibly less time with his son.. but if he doesn't.. more time with an unhappy son?...   I know my DH would have just let the girls do their activity.. it's not like visitation would have been pleasant if he made them miss soemthing they wanted to do.. but obviously, her DH doesn't have to do that.. he can insist on his time and not take him to the game that he didn't approve.. he is absolutely within his rights there.

This kid is a teenager, right? Old enough to know his schedule and relay that to DH? Old enough to communicate how important certain games and activities are?

We had issues with BM enrolling the boys in various activity "flavors of the week" that would result in DH feeling like he HAD to go do these things for them.

Here's the thing about kids: when they want something, they advocate for themselves. Kids are GREAT at incessantly reminding you about something that THEY find important.

Unless SS has come to DH with a schedule, a plea to attend, a plan on how he'll get himself there, etc then DH sticks with his plans. He could talk to SS about how important HE feels the game is, but honestly, if the kids isn't clambering about it, he probably doesn't care.

Let BM squawk. She has violated their original agreement about activities. She has kept SS out of games for other activities. She doesn't have much of a leg to stand on with this, especially if SS is older. While I suppose it's possible that a judge could throw the book at your DH over this instance, I don't think it's going to sway the court decision one way or the other. Perhaps I'm cynical, but my guess is that the judge will offer a pretty standard package on all of this since the SKs don't seem to be in harm's way and BM isn't physically abusing them.

Your DH should enjoy the time he has with them while he has it. There are WAY more important things than a second ECA, and any judge who doesn't see that isn't going to side with your DH anyway.