Whether or not your relationship is good / could return to being good has to be the first step I think.
Because if it's basically good (or could be with a little attention) then it would seem a big risk to leave that and split your DS' family - meaning you see less of him - for something that might not happen. Or for something that even if it happens, makes your existing child's life less happy.
However, it takes 2 people to work on a relationship. So this can't all be you. It needs your DP to accept his part in things not being so great. He needs to accept his change of mind has caused you upset / disappoint and may do so for some time. He needs to accept that where your views have differed on 2 important aspects (another baby and marriage) he has got his way both times.
While it's hard to argue with someone who says they don't want another child, I can't find a good reason for him refusing to marry you. It clearly matters to you, but he's happy so he won't budge. That's pretty uncaring (and aside from the feelings, there are the many practical benefits of being married). That may be a good place to start in therapy - why doesn't he want to get married, when his DP wants to so clearly. What is he frightened of?
If you come to the conclusion that actually he's quite selfish, and doesn't care about your feelings. Or that you can't regain that love you feel is lacking a bit, then go ahead and split. It would be right thing to do, aside from the baby issue.
You will at least split knowing you've given your relationship every chance. You've tried. You may or may not meet someone else. You may or may not have another child. But you will have left an unhappy relationship so you'll be in a better position either way.