Hi Melissa,
UPDATE! Melissa, I'm sure you do feel very low right now. It sounds just awful, so first of all, know that I'm sending you a huge, warm, wrap around you Mama hug! You do not deserve to be treated like this, and I hope you have some good supportive friends nearby to talk to and to feed you chocolate or whatever will help.
THEN, Hon, and this is important, PLEASE think about what you said about how he "took the baby". A 12 week old baby is not old enough to just be taken away from Mama! I don't know if you are nursing--doesn't sound like you are--but that does not change things. If he left with your baby, without any of the normal things the two of you use to take care of the baby, and if you don't go after him (your child) right now, you may just be looking at the beginning of a very nasty custody battle. I strongly suggest you find someone to care for your other kids for a few days while you get yourself to his mothers home right away and find out what's going on with your child. Bring the diapers, clothing, bottles, all that stuff, and use that as your excuse for showing up. He's YOUR SON!!! And if you allow him to simply take off without the equipment, as cruel as it is, your boyfriend and his mom sound like the type who might have a lawyer turn that against you by claiming you are "not a good mother" since you allowed him to do it. So don't. Either get your child and leave, or get your child and stay, but go get your child!
So, doing the math, you have been with him 2 years. Your pregnancy combined with your son's age gives 1 year of that time, half of it, that you have been absorbed in having a baby together and also that you have not had sex, he has been distant with you, and he has not honored special days with you or your other children.
What was it like with him before? Was it wonderful enough to balance out what it is now? Did your older kids like him? Did he like them? Is all this seeming like it is related to your pregnancy and birth of your (his) son? How does he relate to that little boy?
I know just how terrible it can be to imagine feeling alone, and I know how big a deal it is to have someone--even if the love doesn't feel as hot as it was--who is just there with you, helping, working, even just being in the house when something wonderful and funny happens with the kids. But is it enough? Really? Was it enough that if you had back the relationship prior to your pregnancy, you would be happy enough to continue?
I hope you will see a counselor--and I hope he will go with you.