Look at him. Will he appreciate the advice? Start a conversation about something else if you're really concerned. Talk to him once or twice, you know? THEN approach him about it. If its not the first thing you have to say to him, then he'll take the advice in a far, far more willing fashion.
This IS kind of a big deal. Bacteria and parasites and other nasty shit live on his workout clothes.. if he isn't washing them, or himself, then that SMELL is the BACTERIA.. you dig? Human sweat doesn't have a BO smell to it naturally. It's not til it has COME OUT and then DRIED that the bacteria go to work on it.. and that BO smell is the bacteria's BIPRODUCTS.
That's freaking gross. I agree with whoever said talk to the gym manager. Talk to him from the format of "It's one thing to have to smell him - i can deal with that. But he's a HEALTH RISK."
Cause he is.
And it's probably just because when he was growing up no one took the time to teach him the importance of washing himself.