RE: You Stink!!
That once happened to a bunch of us in gym class. A girl just didn't know about deoderant I guess. So what some girls did, which was a bit mean in a way, was buy 10 sticks of deoderant and just "randomly" handed them out, and gave that girl two. But she started using it, and afterwards there was no problem.
If he's not like your really good friend who you can tell a lot of things to, don't come right out and say it. It could come out wrong and offend him. Just slowly imply it and I'm sure, sooner or later, he'll get it.