That is pretty young, but those tiny dirtbikes only have one gear and don't go any faster than a bicycle. There really aren't many choices, so you just don't want to go too big.
The most important thing, however, is not the bike. Get him a PROPERLY FITTING helmet, padded clothes, boots and gloves. He'll be fine. Kids that age do far more dangerous things than buzzing around a track on a bike. How tall are the trees in your yard? :shock: