I always hate it when people use kids as a tool to hurt the other after a breake up or divorce. It is the kid that gets hurt the most in the end.
It is deffinately not a good idea for you to go and talk to her. It will only make matters worse. I do think it's a good idea that your boyfriend wants to try to fight for costody rights or at least visitation rights. Clearly his ex and new partner are not the best parents to be around for that kid and he has a right to look for his son's best intrest. Don't worry about her not wanting the boy around you. She is not the one to decide that, once the court decides that you are ok to look after him. If you could get visitation rights in order this would allready be a good thing.
I feel realy sorry for that little boy and I hope everything works out in his best intrest.