Watch Craigslist. Ask him what he wants. They grow pretty fast at that age so you don't need to plan way ahead. Used kids bikes are pretty cheap, too so if he hates his birthday present then let him pick out a Christmas present. I was sort of operating a back porch bike dealership for a couple of years. You get free kids bikes all the time if you keep your eyes open. I had kids who'd come back every spring to trade up to the next size.
My girls have Electras. The seem pretty solid and have flowers on the seats that match the flowers on the fenders. They also have a lot of adjustment in the seat posts so they'll last a couple of years. $75 was really more than I would usually pay for a kid's bike but they really like them. I hear Giant makes a decent bike, too. I usually say to avoid the Wall Mart stuff but at that age, so long as he likes the color, it'll probably do ok.