Hey,
I feel that those are red flags and you are concerned with good reason. I know where you are coming from, it is difficult to trust a boyfriend who has a history of drug abuse. You have to let him know that your concerns are based on the fact that you care about him and don't want anything to ever happen to him and that you are willing to help him no matter what. Reassure him that you are there for him, but he has to be honest with you. You need to sit down and have a serious talk with him, I know it can be difficult and that it makes you feel vulnerable, but it is for the best.
Best of luck!