I can relate to everything you say but have to ask what you really think, is he using? If you are unsure then what would be proof and would you treat it as an isolated incident anyway?
Finding material in his pocket and a pipe in the car would seem pretty strong indicators.
Sometimes it is hard to estimate the effect love has on hope.
I have the feeling whatever you do where will always be niggling doubts, something he has wished on you, and maybe you have to resign yourself to them no matter what path you take.
One of the things about humans is they have to make decisions without 100% proof, no matter in which field we are talking. It is a question of doing what seems best for all - and using just what you know now.
You are probably right that if you were completely frank with him he might go into a defensive mode, maybe involving lies, maybe anger, or some other way of getting you to doubt yourself. You might find you simply could not go on together after that. Is it still worth a try?
Maybe an air b&b might have a result.
I hope we talk some more
It is not for me to make strong suggestions as I"m not the person on the spot, you are. There is however one thing,at the moment you are feeling you may be making excuses for him , however the point I want to make is you also say "it would be hard to know for sure that he is using".
If it was me I'd think of the future, could there be a a time where you came to regret your actions becuse you did not have complete proof and there was a lingering doubt you had been unjust and the separation -at least for drug use -was not necessary?
Do you think things are bad enough now to separate, or would you prefer to wait till you find incontrovertible proof?
If this last option seems tempting please remember that addicts can go to great lengths to hide their addition. Perhaps doubts in the future are somethng you may always have anyway.