I really don't want to comment on the other part of your post because I have no idea what crack or meth is like... and I know so many people that smoke weed and live productive lives and don't "count their seeds" like I used to count my pills, etc. I don't feel I am qualified to comment on that either, but... I can on the following:
Concerning his "defensiveness". I am the type of person that will get really riled if I'm accused of something I didn't do. I've never bought in to that hoopla that if someone gets defensive, that automatically means they are guilty of what they are being accused. My ex was paranoid and accused me of everything from cheating to this/that and I would get royally angry when he did when I had never even considered cheating on him, he was my world. (And if I was in pain, my attitude was that much worse).
Does he have health insurance where he could see a Doctor without paying an arm/leg for an ER visit? I only ask because possibly that is why he's self medicating. I know with the back, a visual exam by a family Doctor won't yeild much unless there is a knife sticking out of his back, they need tests to help find the problems, but even then--you aren't guaranteed results. One time when I didn't have insurance I paid the ER a visit and with the bloodwork, X-rays, C-scan, I was billed nearly $17,000. (Luckily because I was considered poverty level, I qualified for a program that assisted my ER and hospitalization bills).
Instead of confronting him, why don't you keep a journal (FOR YOUR EYES ONLY) and make note of his sleeping/eating habits, is he struggling when he lifts things? Is he also taking OTC for relief? Does he "go off" on you about anything else?
I'm not telling you to outright spy (well, I guess I am SORT OF, but it's more for your peace of mind) It might make you feel better if your journaling all leads to him being in pain for a real back problem (which you can offer massages, etc), therefor cranky so you don't take it personally and aren't so worried about his sobriety. If it points in the other direction, all you can do is make it very clear that he's a big boy and will do what he pleases but you are there for him should he ever need you... (and take it from there, if it turns in to a problem you obviously can't (shouldn't) enable him).
You posed a complicated situation, sorry my answer is so long and confusing. There are so many ways to look at it.