Awesome :) dads like you @munchell are inspiring :)
He can try out a pewe 50cc at his age, you can fit stabilizers also.
Kids cannot race motocross until they are 6 years old but getting him out on a bike before will give him tons of confidence and also a big edge on the other kids who mostly wont have ridden anything before then. :)
Truth be told, I want him to have something in his life he loves. And if he can make a career out of his hobby all the better. I don't think he would need stabilisers because he never used them for his bicycle and he started off with a balance bike. He is 5 and a half now but he isn't the tallest so we might have to wait a little before he can fit on the pewe bike.
This is him on the bike we got him for his 5th birthday.
Wow yeah no stabilizers needed here :)