While I don't believe your husband should be keeping contact with his EX wife a secret, neither do I believe you have a right to ask your husband to break a confidence to his son/exwife (relating to the son). 

What you DO have a right to know are things that will impact you, your household, your finances etc..

For example, perhaps son is struggling with sexuality.. or transgender... he has a right to "come out" to people on his own terms.  If he is struggling with addiction.. or having personal problems in his own relationship.  These are things that his parents may be trying to support him through.. but if you are not part of his "support network".. and he doesn't want you to know that he is headed for divorce.. it really isn't your husband's place to share his son's secrets.

But, you have a right to know whether your husband is sending him significant portions of money.. you have a right to know if he is meeting up with ex and son on a regular basis. 

At the base of is, your relationship with your husband should be strong enough that you can trust that he will respect your relationship and not do anything disloyal to it.  Just like my husband and I trust each other to act in the best interest of our relationship.. even though we live and work half a country away from each other.  If one of my SD's was dealing with an issue, I would hope that I could be involved and help them.. but if they didn't want me to know.. I don't think my DH needs to share their private info with me.. as long as he isn't hiding things that involve me.

Well, it's not all on the ex-wife, as your husband seems to be okay with it.

is enjoying playing this game with his EX.  It’s really not about there son. It’s about they are talking together for no good reason. Except they want to talk with each other.  This has to stop

Why do you object to them speaking? Do you not trust him? Does it take away from your time together?

Just wondering - I guess I wouldn't care if BM and DH wanted to talk if it didn't really affect me in any way.

This does not sit well with me, nor does it sound right, especially as you said he wants to know everything about YOUR adult children?! First, I don't think the BM should be contacting him at that age unless it's about something VERY serious involving the SK. Second, if it's THAT serious that SK's dad needs to be involved, then YES, as your DH's spouse, you should know what it is as well. That's like with my own BM. If she's having a serious problem, I share that with my DH, who is unrelated to her, since DH and I are a TEAM--what affects one affects the other and we handle all serious problems TOGETHER. And we have no "secrets" between us. BTW, my SKs are now 23 and 25, and BM has contacted DH several times since CS ended asking for $ for one of them, and each time, DH has told her to have the SK contact him DIRECTLY. I'd love for him to tell her never to contact him again, but he feels his way is less likely to cause SKs to get angry with him. Either way, he's putting her off, and that's fine with me. If he EVER told me BM told him not to tell me something, REGARDLESS of who or what it was about, and he agreed to it, I'm afraid our marriage would be over. 

Nope.  Secrets kept from a spouse at the request of an X can't happen. Or at the request of a kid for that matter.

I have kept only one thing from my wife at the request of our son (my SS-27 adopted at his request when he was 22) .  Though I did keep that confidence I regret that it required me to keep something from my bride.

It did not make a difference overall that I kept that  confidence as he ended up confiding to his mom ... and I five years later.

Spouses are equity life partners and those partners to not keep secrets from each other at the request of anyone and for damned sure an X is at the top of that list.  Yes, bioparents should do what is best for their kids. But those kids are also SKids to spouses and the priority of a spouse and marriage is sacrosanct and for damned sure trumps anything related to an X.

Period.

There is no "except" clause in marriage vows to love, honor and cherish.

"I think perhaps respect has a different meaning in step life than it does for the population at large."  True.  It does.  But the dynamics of the 'population at large' is changing.  You are not divorced but I honestly think you would have a different view on this subject if you were.  I am sure that most of the 'population at large' would not mind their spouse having a conversation with an old flame provided it wasn't intimate or unnecessary.  An Ex who calls her DH up about non-emergency subjects is unnecessary.

I see very few reasons the divorced parents of a 30 year old would need to speak about their child. What’s happening in his life that the parents need to discuss with one another instead of discussing with their child? Also, if the ‘child’ wants to keep it private, why is that not coming from him directly?

Trust is not the only important factor in a relationship, respect is just as important. Why the need to speak to BM at all? Unless their kid is horribly sick or going through a life changing event, I don’t see why the parents need to discuss it. Their kid is well past the age where they can discuss with their parents individually.

To me this isn’t a matter of trusting your husband but him respecting you as his wife. I don’t think he needs to share his sons secrets with you, but that, again, should be coming from his son. Not his son’s mother. I wouldn’t be ok with this.

This is a problem for many who refer to adults as "children." It alone implies these "adults" (at least by their age anyway) are to solve their own problems and fly the nest. To me, this sounds like an excuse for the BM (bowel  movement? LOL) to insert herself into the relationship.

My DH and BM have been divorced for decades (over 35 years), and all of a sudden she tried inserting herself into our marriage, by sending my DH a private birthday email. Mind you, she could've accessed him on one of the social media pages, but she elected to send this email:"Good Morning_____. Today is your birthday, and I've always paused to remember that and reflect on "us" on this very special day. Your father was an amazing man who lived well into his old age, and I know you will too. I just wanted to let you know you're on my mind and in my heart today and always. Please don't mention this to DW, as we have had a long history, and I just don't think she would understand what we have."

Now let's carefully think this through. 1.) It didn't involve a "child." I'm sorry if I'm comment bombing your original message, this is not my intent, but rather BM's contacting DH's privately. 2.) She had no right to contact my husband privately, and for her own seemingly manipulative and self-serving reasons. 3.) This has nothing to do with me being insecure, worried, etc. (I'm 20+ years younger than her, and DH has told me it was his choice to leave her, as she was a slob who never cleaned house/laundry/or knew how to cook in all those years), and has assured me she was one of his greatest mistakes, and unfortunately he had his kids with her,whom he, of course loves).

THe simple fact she is calling and specifically asking your DH to keep it between them is an attempt to triangulte your marriage. You must guard not only your heart, but also guard your marriage from ANY 3rd party intrusion to it, no matter who or what it is. If is smells like BS (BM, bowel movement) than it can be  nothing other than BS.