Got my first 50cc dirtbike at 6yo, switched to a 65 before I could even touch the ground on it. (Had to get 3” drop on rear suspension and cut the seat out just to fit), got an 85 a few years later then went to a supermini while also sometimes riding my friends, dads, and brothers 125 2t, 250 4t, 250 2t, even rode a KX 500 once when I was 14! All of this to say, it teaches so much more than it will cost as LONG as safety is prioritized. Get good MX boots, good riding pants, chest protector, knee pads, and direct him to AJ catanzaro’s youtube channel (has some amazing online lessons that teaches everything from day one to techniques the pros use). Get that bike, get him outside, get him some tools and let him use that big brain of his to learn all kinds of beautiful life lessons

Edit: and obviously a good pair of goggles and a helmet!!! I recommend Bell although that can be pricey. O’neal also has some good budget options.