Even as someone who is for decriminalising drugs, I can categorically say this is an extremely bad idea, and you should walk away immediately.
I was in a similar situation last year.
I gave him a shot after learning he was on methadone for heroin withdrawal. He also said he had been doing cocaine.
He stopped for about a month, and then it just escalated. Nasty temper. Turning up drunk on top of everything else. Snapping at me and saying nasty things. Then it got physical.
We are not together anymore. I did a Clare's Law request, and what they told me about him? He should be behind bars.
This is not to say you shouldn't give someone with past drug issues a chance. People can and do recover.
This is to say that he will inevitably start to deteriorate, and you'll start to notice it, and soon it will start to pick away at you too.
So nip it in the bud, now. He's not quite at the tipping point yet, but he will be soon, and you should get away before he pulls you down with him.
He might be amazing if he gets clean and stays clean. But right now? He's a bad idea.