Most likely he was being nosy
Such a disappointment. It could have been many things, my bet is he was just being nosy, a lot of people go through other people's medicine cabinets and all that.
But regardless of why he was doing this, it is going to hang between you. If you don't talk about it, he will always wonder if you saw, no matter if you acted like you didn't or not. You will always wonder just why he was doing this. Any reason I can think of still adds up to a lack of trust. It is the beginning of a wall between you, on both sides. That is no good.
The easiest thing would be just to write him off. Most people would write him off. If you don't want to do that, then you and he need to have an honest heart to heart. See if he owns up to it, what he says. People don't have to be perfect but trust is so important and most of us have trust issues already.
But if you don't have that much time and emotion invested yet, why bother? He has good qualities, too, but so do a lot of other people. Talk, if you want to still give it a try.