Yes...they smell ripe! Just walk through a gym class in the afternoon, with a bunch of 3-4th grade boys. Eeeeeeeeuuuuuwww!
It's great you are having this talk. Just be matter-of-fact (kind to his embarrassment), and bring it up during a quiet time. Try not to show your embarrassment if you have any. Conversation... "Since you're at the end of 3rd grade, I think it's important that we start addressing hygiene... at your age, it's good that you take showers once a day, and in the morning you should be putting on deodorant..." He may be totally embarrassed or the cool kid that knows more than others. It's amazing. Let him know that you can pick it out for him but it would be better for him to choose his own scent. We usually go to Target for that kind of stuff. This conversation was easy for my son - he hated the way gym class smelled and didn't want to be part of the problem!
Yes, teen brands are a good start. Buy "deodorant" NOT "antiperspirant/deodorant" because the latter have an aluminum based product that can actually clogs our pores and that's counter-productive to what our body is supposed to be doing. Same with "antibacterial"...products. They carry chemicals that our body just doesn't need. Good hand and body washing, with regular body washes or soaps are sufficient. He may want his own body wash too - he doesn't need a body spray but by 5th grade he may try to convince you that he does.
Currently, my son likes AXE deodorant (which is not a teen brand but they have "deodorants" as well as "deodorant/antiper"... so watch your labels. Some of the AXE products have such a strong fragrance. He uses "phoenix" and has used "clix" but you just have to smell them to see what appeals to you and him. Then tell your son to "just swipe well, once", otherwise, he may smell like a rose when he gets to school, as a little goes a long way.
Good luck.