I have a KTM 50 sr. way too big for him now.Put that boy on a real motorsickle!!
Me and my cousins were riding a little Indian 50 that all of us grew up on right around his age.
KTM makes a 50cc 2smoke centrifugal clutch bike. My son had one around 4 or 5 years old. Had him flat tracking mad dog XR100's at 9. lol
Start them early, Never use training wheels, and as soon as they are able to get their throttle control and braking and the basics of riding, get them a bike with a clutch. I know all the "cool kids" are doing rekluse clutch, but its the best way to get them to be good riders.