Well, normally I'd start him off on something in the 100cc range, but since he's kinda tall, I'd put him on a Honda CRF150.
Docile, light, great fun to learn on, and prolly the most reliable motor out there. Much more beginner-friendly than any 2-stroker (IMHO).
My sons both went thru Honda XR / CRF 100s, and Yamaha TTR125s, but my sons were more like 5'-2" ~ 5-5" when they were that age...
Enjoy!
Kirk