I kinda get the idea that they don’t really have the room for a dirtbike where he’s at. and the worst toy is the toy you can’t play with when you want.
one of those pocket bikes can get going pretty quick but they can also be a lot of fun. I figure as long as he’s responsible enough that you believe he won’t be taking it out when you’re not home and he’s unsupervised then it should be ok, remember as kids we could get pedal bikes moving shockingly fast too. keep him off the streets, and out of traffic, and you’ll be fine.