Sorry, OP, I have to agree with all the other PP's.
This relationship isn't going to go where you want it to go and he's being utterly selfish and cruel to string you along as he is.
It boils down to one question-how long are you prepared to waste on this guy?
I think what you're holding onto right is a dream. A fantasy of who he could be and not who he really is. You're scared to let go because there's always a chance, however slim, that he could change his mind and suddenly want kids and marriage.
This isn't what he's showing you and can you afford to waste more of your life on the off-chance that he changes his mind? What if he doesn't?
And let's say he does propose, you will always remember his hesitancy and wonder if he's just doing it to appease you or if it's a "shut up ring," or because the woman he really wanted didn't come around.
I don't like to analyse people's thoughts like that-everyone is different-and he may have genuine concerns around marriage/kids-however, it's irrelevant. He's not giving you what you want. That's all that matters and now you need to work out what you do with that information.
If it were me, I'd have a final conversation with him about it and if his position hasn't changed-I'd be leaving.