You already know the answer to your question. Sometimes we can't help who we're drawn to, but the beauty is that we have intelligence & free will to make the decisions that, ultimately, are the right ones for us.
This guy is a zero. You know it. He knows it. He, as a whole ton of people on this planet, would like to have his cake (live with FIRST baby-mama) and eat it too (fool around with you). Seems like a pretty good deal for him, right? At the same time, it's a shitty deal for you AND your kid.
If the 2 of you don't work as a couple, that's one thing to explain to your child. "Sometimes people just shouldn't be in a relationship with each other, but that doesn't mean either of us loves YOU any less." If you continue on the road you're leaning towards right now that conversation will be a lot more like this: "I know you father uses me, but when you grow up I don't want YOU to act that way." See that? No good. You can't explain that to your kid & trust me, he'll be catching on long before you think he's capable of getting it.
Stop fooling around with this guy. When he says he doesn't want you to be with anyone else, but at the same time tells you that he's moving in with his ex, tell him that it's really none of his business what you do with your personal life because that's the honest-to-God truth of the matter. Discuss stuff about the child you have together only. You don't owe him any explanation, you're not currently in any sort of committed relationship with each other. Even if you were, honestly, he doesn't deserve an explanation after treating you the way he has been.
Date when you're ready to date. You'll know when that is because you'll find yourself scoping other guys out & when someone asks you out, you'll see that you actually WANT to go. There's no need to push it. You were in a 10-year relationship that broke up, you had a child, your ex consciously screwed with your head. That stuff takes some time to recover from. Cut yourself some slack but for pete's sake, leave this loser alone!!