Agree with others that say the first issue to be handled here is the fact your DP hasn't bonded with his child and in fact resents it. That is unacceptable behaviour in my book and there's no way you can consider bringing in another child to be resented and ignored.
Was he like this with his older kids?
I don't think you can compromise when it comes to kids, especially if someone changes the rules. My sister spent all her reproductive years with a man who wanted kids, marriage etc, as time ticked on she started to raise it with him, turned out he changed his mind and decided he no longer wanted kids as they were 'too messy'...
She left him, but didn't meet her current DH until she was 38 and they weren't ready to try until she was 40 and it never happened, she is now childless and my heart breaks for her every day (even though she is the best aunty you could ask for and is incredibly pragmatic, never dwelling on it, she just would have made the best mum and it hurts me that he 'changed the rules' and it is her who suffered (needless to say, said ex then hooked up with a woman who already had 2 kids...)
If your bloke claims he's never wanted more than one with you (and has no interest in his current baby) I can't see that changing.
But, unless he gets help or starts taking steps to bond and not resent your boy, I'd be leaving him anyway, this might sound melodramatic, but there's no way I'd stay with someone who resented a baby - for me, that's something I just couldn't get past, how could you trust them to give your baby the care he needs, if you know he wishes he wasn't there?