Ive had all kinds of toys for kids brother. Youngest a new adult now. Go-carts, 4wheelers, dirtbikes, and more. By far the most ridden and over all safest thru the years, has been the dirt bikes.
Others will disagree, but bikes do not chase you down normally and crush you when you crash. You go one way, they usually go down or another. Ive ridden all since I was a young child also. My sons first dirt bike was a yamaha 50 at 5 yrs old. at 8 he was on a TTR-90cc yamaha KTM-85sx at 10, and this little shit jumped to riding my KTM SX-250, when he was 12. He is now 21 and rides a GASGAS MX250 in the mountains and does some pretty knarly single track rides, of which I "used to do". I ride a GASGAS EC-300, woods weapon set up, awesome GPR steering dampner, flywheel weight for those long hard steep hill climbs. Anyway, I digress.
Point is, my 2 girls have ridden and my son rides hard and for years. The only injuries that any of them have had that required a visit to an ER, were from the go-carts, and the flippin 4wheelers. Crashed the bikes many times, but with proper gear, its never been an issue.
Last summer I met some people from Pennsylvania, they came here to visit. Brought their 2 kids, one a 7 yr old spolied little shit boy that wouldnt listen. I seen the dad letting the kid ride with out a helmet or chest protector. After hearing my advice, I was dismissed easily. I told my girlfriend I wanted to leave her paremts and go, BEFORE the kid ends up dead or hurt.
3 hours later her mother called to say they were headed to the ER. The kid ran into a piece of farm equiptment near a bailer. Metal piece pierced the 4wheeler plastics and thru the gas tank and into the kids belly. Ended up with just stitches thank god. But proper gear would have prevented it and rules for the kid. But if it had been a dirtbike, he would have been able to avoid where he was and turn correctly.
I say get the boy a dirtbike, he will have the time of his life. Enjoy and have lots of fun