If your therapist is going to think you are the "bad guy" because you are upset about your SO exchanging 100 texts with his ex over one problem with their child, you need a new therapist. Honestly, if it was that big a deal, they should have actually talked about it instead of texting. It could have been resolved much quicker. It is not normal to exchange that many texts about any one subject with anyone - at least in my world.

So.. it was 100 texts.. were those 100 HE sent.. or was it a mix of he sent/she sent?  and if so.. how many did she send.. and how many did HE send?

I guess he falls into the trap of wanting to have the last word.. texts are easier I guess because you don't have to "face the other person" in the same way.  a text can also be done very quickly..

I mean.. lets say that it takes 30 seconds for a text. that is like a total time of 50 minutes.. total.. so it's not necessarily the time suck but perhaps being upset about the issue at hand is making him less productive?

In the grand scheme of things.. they probably would cover the same ground in 100 texts in one or two conversations.. each text is one person's point.. that will be countered in a discussion.. would you prefer they met to talk about the son personally?  I'm guessing not.. 

Part of the problem here is that they aren't communicating effectively.. and THAT is the problem that is leading to the multiples of texts.. when she starts blaming him for all the ills of his son's life. he has to learn a way to shut it down and not continue to engage in that with her.  She is saying it to get a rise.. because she is frustrated.. because she wants to strike out at someone.

Based on what you have posted here in the past.. I think both his parents have had a healthy influence on him being a spoiled and entitled teen.. and that is probably not the kind of attitude that is going to gain you friends.  

AND... teens go through these kinds of interpersonal and social upheavals.. they break up with friends.. they form new alliances.. as they figure out more about themselves.. it's a learning lesson for some too.  If you are being cut out.. examine the situation.. did you contribute?  or are they people you would rather not be associated with anyway? His parents can be sympathetic but they can't necessarily fix the situation for him.

I do think that therapy is a moot issue for you though.. because there is almost no level of contact you would be happy with.. and certainly 100 texts is a lot.. but the parents feel they are in some crisis.. and they aren't thinking 100% straight on that wavelength.. and he will listen.. change for a few days maybe then something else will come up.. that he will have to be involved in and you will be mad again?  

Because you have zero tolerance for their BS.  You are hurt, angry and raw to it all.. and there is going to be about zero chance that any amount of therapy is going to resolve or improve anything.. I mean.. you both went before.. and he flatly ignored what the therapist told him.. why would this be any different?

Your husband likes catering to his kid.  He likes the communication from BM about his kid because it makes him feel involved in his son's life.  He resents you trying to minimize that contact.. this has zero to do with kid talent. or even bm wanting him back (I don't think she does).. it has to do with a guy that truly believes and enjoys being a father in this way.. and that is absolutely contrary to the kind of man you want to be with.

So.. you can waste more time and money on therapy.. which isn't going to change a thing.. or you can get your plans moving to get out.. 

If BM isn't explicitly asking a question, then he doesn't need to respond. She is trying to keep him engaged in an ongoing conversation and your DH complies.