J.B.
Hmmm -is he into any type of sports? A lot of "fan wear" -athletic jerseys, sweatshirts -that type of thing is made to be worn kind of big and loose and comes in all sorts of sizes. It's colorful and fun. You could get him an adult M or L. At his size, you should probably be looking in adult and junior clothing sections. You may need to hem his pants or jeans from those sections, but he's not "child-sized" anymore.
I know you didn't ask about his size so much, and I know he needs good clothes to wear regardless, but has he seen a doctor about his weight? He's at the perfect age to really learn and start making healthy lifestyle choices that will help him the rest of his life -eating low fat, unprocessed foods, reserving sweets and "junk" foods for special occasions and treats once in awhile, and possibly the most important -regular exercise! Find something he really enjoys and sign him up for a little league team or if he likes biking or skating, perhaps you could do that often as a family. Hike in the woods every weekend -that sort of thing. Good luck with everything!