I'm interested to know how or when you found out he was a user and did you feel completely betrayed by him hiding it?
Also how are you now spending time with a lot of heroin users? Are they his housemates or his friendship circle? The situation seems to have gone 360 from being oblivious to now being surrounded by it.
The bottom line is to stay on your guard. As you've discovered, addicts are liars and extremely secretive. I agree, addiction is a disease and should be treated as such but the lies and manipulation are part of the illness.
I've used drugs and would tell myself every excuse in the book to use. Addicts lie to themselves more than anyone else to justify things, whether thats taking drugs, refusing help, or justifying borrowing money, taking handouts. It's all deeply entrenched in denial and lies.
Do think carefully about getting in too deep with this guy and emotionally attached. Not just to protect yourself but because it's difficult to remove very involved emotional support from a drug addict without it seriously affecting their fragile mental health.
I understand you're heartbroken by the cruel hand he has been dealt in life but that shouldn't be a driving factor to becoming romantically involved.