I agree with previous posters. Don't think so much about testing at this age, but rather about providing stimulating experiences. If he's reading, make sure to provide plenty of interesting things at an appropriate challenge level. If he's interested in math, get him some manipulatives and appropriate games and toys. Take him to museums, etc. If you can find a preschool that will provide appropriate stimulation and isn't afraid to place him with older children, it might be a godsend, but you might have to be content with just socialization for now on that front.
