Well first of all, no it's not cheating as you asked in your title. Of course, be careful about relying on some other guy for emotional support it could turn into an emotional affair at some point. I understand the need to be distracted and have a friend. After all you're only 21 and you're dealing with a drug addicted. IMO, he already fell back into it, if it were me, you're not like married to the guy, I would be wearing thing tbh of my patience. On that note if it's a long time, I don't know if I'd be able to wait, you should have a devoted bf that would be drug free and/or give it up if he fell into it, for you, in addition to for himself. I'd give it some time but don't wait forever for this guy. you're too young for that.

On the subject of not telling him. Do you really want a long term relationship based on keeping anything from him? Would you want him to keep things from you? Regardless of if it upset him you're not doing anything adulterous at this point and a trusting loving bf would accept this. I think you need to be upfront with him about that or don't do it. Make the choice that way. If it's something you'd have to hide, then don't do it. If you can tell yourself you'll be honest with him about it then go for it. I just don't think that's the makings of a strong relationship.

My rule of thumb is, if you feel like you have to hide something, even if it's not "technically" wrong, it's probably best not to do it. but that depends on how far you'd go for the person you love...