Maybe He's Not Ready
I agree. I don't know what your son's size is, but do the math to compare ratio of bike to rider. Then figure out how heavy of a bike it would be for you to ride. Then add another 30% for lack of muscle, and another 30% for lack of experience (any sports, not just riding!).
Don't rush him. Let him get proficient on bicycles first. Everyone (including me) thinks their kid's a prodigy. I think training wheels are a bad idea. You can't lean into the corners, and if you do, you'll crash.
Also, a PW is a lot lighter, less powerful, and easier to ride. He'll grow out of it fast, but if you must start him, that would be better. Do the ratio calculation on that one, and it will come out a lot better.
Let him want to ride, then turn him on to it.
Regards,
Dear Abby