"...I keep thinking we could be a happy family if only he'd give up the drugs and get help with his mania."
This is so sad. . The "if only" keeps people like you (and me, a few years ago) stuck in these relationships with addicts. What you need to try to do is to forget, for now, what you think the relationhip should/could be like "if only" he wasn't the person he is today. You can't know that. He could stop smoking dope tomorrow and still be a self-centered asshole. Or you could spend a decade waiting for him to stop.
Instead, look at what you have right now. What are you getting out of this relationship right now other than the hope that, in the future, he could change and not treat you so shabbily?
The bottom line is that, right now, you are effectively financing his drug habit. I'm afraid that I very much doubt you'll ever see that £4,000 he owes you I'm afraid.
Right now, he doesn't want to stop smoking. Why should he? He's got someone to pay for his bills so he can spend more of his money on drugs, he's got someone who cleans the house, cooks his dinner and looks after his child, and he can sit around stoned all day. However much he may or may not care about the effect of his behaviour on you, he cares less about you than about drugs.
The thing to do with an addict is to ignore what they say, and absolutely ignore any promises about what they're going to do in the future about their addiction. Instead, pay attention to what they do. He's doing a lot of drugs. Do you want to share your life with a drug addict?
As for his family, it is absolutely not your responsibility (or theirs) to try to fix this man. His drug addiction is his responsibility and nobody else's. Your responsibility is towards you and your child.
Raising a child with a drug addict is wretched and potentially dangerous. If Social Services found out that there is a habitual cannabis and cocaine user in the same house as a baby then they will not be impressed. It may well be worth you talking to Women's Aid.