I really don’t think that him taking the tablets when you don’t have sex means he’s cheating necessarily. You are bang on that it’s the trust at issue. I really feel for you. You’re tying yourself in knots here and as you say, innocent things take on a different meaning.
I know you’re worried about bringing it up and also feel you’re doing the heavy lifting here, but you will need to do something at some point. Because your worry about it all may well eat your relationship up. It was going that way for me.
I don’t know if you’re tempted to confront him, demand the truth. I mean you deserve the truth, so it’s very tempting. But is it possible to visualise, even rehearse the conversation a bit. What you want is a happy, respectful and honest relationship. He’s not doing that at the moment. Perhaps you could show him how it’s done?
He’s already left the tablets where you can see them so maybe start with that. Could you say that you don’t have experience of viagra? And can he tell you more about it? Then you’ve at least opened the subject up.
I have found with my dp that I need to raise a tricky subject with assurances that we can work anything out. And then I back off and give him time to cogitate a bit on the next bit of the conversation. It can be weeks and occasionally months before we have the next bit of the conversation but if I push it, it kind of turns into an inquisition 
I find that opening the subject up and giving him time to come back to me helps.
He may well feel defensive and quite possibly humiliated. And it might be more productive to let that initial reaction subside before you really get down to how this is impacting on your relationship.
I’m only going off my own experience here remember. Your relationship will be different to mine. It’s just a suggestion on getting the ball rolling.