First of all, when he comes to the room, don't say it immediately. Wait while he settles and then come to the point.
Now say like,
I think you are not aware of that odd smell, but I'd like you to know that it might be your feet.
Sometimes, person in front doesn't know what's going on. Therefore, you should be clear and concise
After that, if he notices the smell and washes his feet, then bingo. Problem solved.
However, be prepared for denial and excuses. You may listen to him, but don't try to get caught in the argument. Try to repeat it couple of times for a couple of days. If it still doesn't help, be firm if he denies and try to leave the room for a while.
My roommate during college had the similar problem, so telling him that a couple of times did help and later it became his habit and started to wash his feet whenever he comes from college.