Oooh great subject!
That's a very touchy area in our culture, talking about one's bad body odor. It could make it even more awkward if you've noticed it for some time and now decide to bring it up. He might have tried alot of things and is very self-conscious about his problem (he probably already knows he has BO but is feeling helplessness related to no effective treatment). What you could do is show him he is not alone in his problem possibly by acting like you have BO yourself, and telling him what works. If he insists he doesn't, give him some tough-love and tell him he does.
Bacteria on the body eats sweat and their by-product is what produces the odor. No amount of a deodorant can fully mask this.
What he needs is a very strong liquid Anti-perspirant. I use Certain-Dri (Aluminum Chloride is KEY, he can get a prescription that has up to 20%, but the higher the concentration, the less compliance related to underarm irritation). It will take about 3-5 days to take full effect. It needs to be applied every night at bedtime to DRY underarms. This is not the anti-perspirant you use when you step out of the shower or something you apply "a little extra of" just before you go out. Applying it that way will do absolutely nothing. It absorbs into the skin and he can take a shower in the morning. He can follow-up with any regular nice smelling deodorant in the morning too. The goal though is to treat the sweat under the arms, without that, there is no odor.
Hope this helps