How do I get my boyfriend to take on more responsibility with my son?

I suggest that you go to family counseling to work things out. Having a third party involved really helps. If you don't resolve this soon your problems will continue.

Run like hell! He will never take on the responsibility you want him to. Your child will suffer greatly from this, especially when he is old enough to understand. You need to just take care of your children and don't date anyone else until they are grown. You already know that a piece of paper isn't going to change anything, no matter what he says. He is a looser.

Please understand that not every man finds it easy to take care of someone elses child it is hard to bond with another mans child and that is a fact however that does not mean that it cannot or will not happen. you mentioned that you and him have a child together, of course he will be closer to his natural child but if given time and not pressured into anything he will grow to love and treat the first child like his own. I know this because i am going through the exact samething and i am the man
accepting my girl's child. It will not happen overnight but it will happen you have to be patient because above all else that is not his child and unless he legally adopts he has no parental rights so if the real father deciedes to get in the picture, the boyfriend has to stand aside i hope this helps.

You have to be very careful both ways you look at it can bother you because you are a mother to two children biologically i have a very good friend who entered a relationship and also had a child with and before the man.All is well now and he treats them all equally. He may just want to spend more time with the new born it is so fun getting to know something you helped create, but you older son also needs alone time with you both as well don't let it be an excuse as to (oh well when we get married it will be better) big NO fix it now before