My heart goes out to you; it sounds like a really lousy situation and that you care about your BF and his son ...
Why not DV and then dispute as "not mine"? Search here or on the other boards about the one-two punch. You may be able to get this deleted. I'd love to see that instead of your paying it. Hope you have pulled his reports (all 3) to see if the ex-GF has caused any other nasties ...
Good luck, and keep us posted.