My son is going to be 3 april 27, he is 38 pounds and 38 inches tall, what kind of vitamin should he b taking?

Vit D. Dr charlon's answer is correct. 1 caveat. About 1/3 of kids ahve been shown to be low in vitamin d levels. Altho not totally clear why. The american academy of pediatrics has recommended a vit d supplement for preadolescence of 600 iu per day.
None. If he eats healthy food like veggies, fruits, beans, nuts, seats, whole grains, unsweetened almond milk and wild fish like salmon, he'll get all the vitamins he needs. 15 minutes of sun a day would also be good.