You need to provide a little more info. His age, what he will be doing with the bike and your budget are important.
If he's very young or simply wants it to ride around the house an inexpensive bike will do. if he's older and wants to use it for jumps and trick riding a <$100 WalMart bike won't last long. Good BMXs that can take a lot of punishment cost $200 - $300 and up.