You don't even have to put your face right to it. If you even get on that level you can smell it, or if he sits somewhere for an extended period of time the seat smells. Once you lent him a pair of pants and they smelled bad afterward.
You don't know why; he showers and is not a particularly dirty person. What do you do? Would you bring it up, and if so, how? He's really sensitive about things like this.