Dear weird
When someone posts here you do only get snippets of info ( & not all the facts needed)
YOur ex has anger issues ( so no not a good idea to involve him ) he will react with anger & your son will only shut down
Soz to be blunt ( could the emotional immaturity be caused also by the witnessing of abusive behaviors towards you ?) you don't have to answer this here ( just something to consider & talk about with another eg- your own therapist if wanting to )
I'm also wondering is he also mimicking behaviors he might have seen btw you & your ex..( was he witness to abuse? eg- verbal angry outbursts / put downs / intimidating behaviors/ control of you? ) Maybe hense why the no control issues might be emerging now to perhaps? having seen your ex doing the same intimation / abuse of you?
Could the Lack of maturity be stunted growth due to anxiety of seeing what he has growing up with you 2?
Another thing to think about ( if you do decide to tell the bf ) might be to also ask him to be a source of good male bonding / role model ( you would have to gauge this though ) as your son might instead resent the talking to by the 'new' guy in your life ..( so you would have to weigh this up before trying also ) ..
( instead of the anger issues your ex might be displaying
Eg- you have this chance to help ( as the therapist is correct ) he might end up not being able to control with the wrong person ( & be charged which something)
( & I'm soz to say might this then also be the end of his life ) as he would not maybe survive also in jail either...
So you have nothing to loose ( try your best & maybe all angles ) right now even if it doesn't work ( if you love him & want to try ) try now before it is to late... It still mighten work but even if not ..you will be able to walk away abit more easier knowing in your heart you tried all you could for him..
So maybe restart again ( after Mondays therapy session)
(start to teach again with a male present ) & also maybe he might even offer support in the form of a backup verbally)
( start teaching him the correct / law abiding 'rules' again ) drumming into him the right & wrong way of being when with a gal & EXPLAIN it clearly ( in easy terms )
Also write out the things you need to say to him care fully before speaking to him
Repeat if necessary in small doses ( it might just be enough maybe also to stop & make him think if in the situation alone with another person ) not to go ahead ( you can only really hope so ) ..
If the bf can't ( or you don't think it appropriate) maybe another male relative / close friend (willing to help ) ?
The idea being to show him that a woman needs to be respected ( & that NO means no ) go through in DETAIL of different senerois with him ( lay it out in black & white for him ) tell him all you can ( in small sessions also if you think more appropriate - eg- due to attention problems )
You could only try & see how it goes ..
Coming from you 'lessons' might be a way to help also ...