I’m so sorry you’re going through this. For what it is worth, when I was 5 months pregnant I discovered my partner was lying to me and sending inappropriate (but not overtly sexual) messages to another woman. I tried to move past this and didn’t tell him that I knew. But of course his behaviour continued, and it turned out there were loads of other ways in which he wasn’t good enough for us.
He has made my life difficult for years and years now and I bitterly regret not ending it when I first found out he wasn’t the man I thought he was. When someone shows who they are believe them and all that… At least by myself would have had far more autonomy and been able to focus on myself and my child without him dragging us down.
I say this as someone who already had a child and had been happy as a single parent many years before I met him.
What I’m saying is - being a single parent is tough and the way he has treated you is despicable. But my experience is that if you can accept the new reality you will be better off than tied to an unhelpful (unkind) man.
Try to think of and welcome all the chances you will have to make special memories with your son, without having to fret about your ex, or compromise with him, or be worrying about what he is up to. Imagine the gift of not letting your son grow up thinking playing video games and shirking responsibility is ok and “normal” for a dad.
Try to reach out to friends and family in real life to build your support network. And be open to friendships later, even with (supposedly) happy couples. lots of people’s lives are not the way they appear from the outside. And just because someone isn’t in your shoes doesn’t mean they can’t empathise with you or be wonderful friends, but this is much harder online than it will be after you have an actual baby to take places and play with and talk about.
Show your son how strong and capable and loving women can be. You don’t need a partner to do that, and do a brilliant job of it.
good luck!
PS parenting is hard, even when there are two people etc. It will be hard. But it’s way harder with two ‘children’ to look after.