There’s a mix of responses here.
I will add mine, and hope I don’t offend anyone with addiction in their families/lives, as I really don’t mean to.
I had an ex, OP, who was not an addict at first, but was a cocaine user. It started similar to how you say, it was his ‘friends’ who influenced him. And I believed that for a long time.
There was no trauma or anything like that, he just used drugs on weekends with his friends, and it progressed and progressed until he was doing cocaine on Tuesday afternoons and had lost all perspective, would tell me that everyone did drugs and it was just me and my friends who didn’t and we were the weird ones. There was a lot of gas lighting and horrible things and it almost broke me, it’s only all these years later that I can see how badly I was lied to and how much I blindly believed and put up with because of that belief. Honestly, he was an absolutely terrible person who did terrible things to me (emotionally not physically), and that started before it was an addiction. At first for him, it absolutely was a choice and the choices that followed were awful. At some point it did turn into an addition, though he’d never admit that and I’m sure to this day ten years later he never has.
I am so sorry that you’re having to deal with this with a 6 month old baby in the mix. I hope that this is a wake up call for him and he stops it all right now before he loses you both, but I think you really need to make some hard choices right now, and not believe anything he says. You know him better than anyone on the internet, I think you’ll deep down know if this is a recurring problem.
Take some time for yourself, reflect on what you want your future to be, and try to remember that no matter how much you love him, you have to think of your baby now.