I think Peppermint said it well - glad your story turned out well, PP 
There's no saying whether your partner will decide to grow up or choose life on the lash, Always. The point, as you well know, is that he doesn't even see the choice is real at the moment.
Therefore the ONLY way forward, for you, is to close him out. Accept - very sadly, I'm sure - he's a great play pal but a drain on resources when it comes to family life. You and your DS absolutely deserve better. Start imagining your cosy, planned future as a stable little family of 2.
When you're settled and everything's working well for you single, you'll be much better placed to decide whether you'd even want him back at that stage, transformed or not. And, if it becomes a possibility, to be utterly firm on your conditions. Because, by then, you'll be secure and sorted :)
You've mentioned other issues in your relationship. This makes me think he perhaps disrespects you in a lot of ways. Again, it'll be far easier to free yourself of any sense of "having to" put up with crap - from him or anyone else - when you're free to build your self-worth up in your own way.
I'm really hoping the new year will be one of freedom and positive change for you.