I suppose at the end of the day it's his choice that for what ever reason he hasn't taken up the extra contact. Also I think you are trying too hard to make ammends and are worrying about something that you have very little control over. It is your boyfriend's situation I suppose. He is the one that has chosen not to have him more if he was entitled more.
The situation with the brother is a bit strange. why would he get himself involved like that unless your OH said so at the time? Doesn't make much sense. I suppose that happening didn't do you OH any favours.
I think maybe you need to stop caring so much about the situation. You can't let this take over your life. You can't change how his ex acts. She is resentful and sounds like she just wants rid of your ex for good but has to maintain contact for the child.
Regarding having communication with his ex, you don't really need to, sometimes it's easier to avoid conversations with ex's because they just want to argue with you.
its best if there has to be communication between the two parents that it is purely about the child or maintenance or whatever. you don't need to have commmunication that involves chitchat. What about just sending the odd text or email. is she willing to do that?
There isn't much you can do to be honest apart from being supportive to your OH. Its more up to your OH and his ex to meet and sort out their differences. Very frustrating to have little control in the situation.
Also I agree I don't think she is jealous. Probably she just wants to move on and start again and the harrassing has left a negative opinion of your OH. I think he should work hard on being polite and supportive to his ex. And providing money would be also a good start in the right direction. maybe he could ask her what HE could do to improve the situation between them. How could a better atmosphere be achieved? What does she want out of the situation? what do you want? and thirdly what does your OH want? and what is best for the child? And how can you compromise?
Is your OH rude to his ex? just wondered. maybe he should think about what he says to her? Maybe he says things she doesn't like so she feels the need to cut him out.
Thats all i can think of at the moment