To be honest I shouldn’t be mentioning the cheating because that could upset you if it’s not true.
It’s just a seedy drug and makes you do things u normally wouldn’t do. And you do a lot of sneaking around ect..
Just sit him down and have a nice quiet word ect.. And say will he do a test just to give you piece of mind and you won’t mention it again. If he says no he obviously is using.