You know, that's just one of those common fallback excuses used frequently when a person is simply NOT interested in marriage. And, bluntly put, WHY should he want to get married; he already has HIS "marriage". You're already in his home, in his bed, and have already reproduced with him. Where's the "need" for him to get married? What more could you possibly bring him through marriage than what you've already given him??? Now for him, you can take more with marriage. So, "what's in it for me" needs to be answered for him.
I guess if you want him to value you to the point of marriage you first have to value yourself to the point of marriage. So far, with your actions and choices, you've activily campaigned against it and are now trying to change the rules of the game.
Stop taking excuses from him. Stop MAKING excuses from him. If you want a life long emotional, legal, loving relationship you have to market yourself for one and stop buying what you don't want. I'm certain he's a wonderful guy, but he may not be husband material for you (or visa versa), so you need to decide what you really, really want and why. Once you do that you have to follow through with that completely with 100 percent integrity to your choice.
I don't know how else to say that. I really don't want it to come out harsh or hurtful, however; that is how I see it and I believe the best way to tell it is to simply say it and allow you to weigh it out for what it's worth to you.
Best of wishes to you Sweety, these things can be difficult to work through when you're right in the middle of it, the best things are worth working for and waiting for in their proper perspective and place.
Congratulations on your baby boy!