Zumiez is more of a skateboarding 'fashion' shop. Not ripping them, just saying that selling decks is secondary to what they do. Are you in Portland or SF? I can help you find a better local shop.
Do NOT buy a cheap deck. No reason to. Your not talking about a particularly expensive item, and a cheapie skateboard will be just terrible. Poor bearings, low quality wheels, and the trucks (the metal bits that connect the wheels to the deck) will not be as stable.
Decks come in three flavors these days, standard (freestyle/trick boards), cruisers, and long boards. Standard boards are the most agile, but they're typically going to have smaller, harder wheels, that are not as nice for cruising. Cruisers are larger, still capable of doing doing some tricks, but more geared towards sidewalk surfing. Larger wheels and a wider deck make for more comfortable cruising. Long boards are just that, long, and are for down hill cruising. Cushy and comfortable, but also big and heavy.
I think a standard with softer wheels would make a good start for your son. Lots of shops will let you pick out the gear you want (wheels, bearings, trucks, deck) and assemble it for a nominal fee. A decent shop will also have decent pre-made boards.
If you son wants to go to a skate park, tell him he must wear a helmet at all times, period, no discussion. You also might want to look carefully at who else is in the skate park. There are plenty of reasonable people that skate, but there are also some lost kids who are not up to any good who hang at skateparks. I know, because I was one of them in my youth.
My decks below, standard and a cruiser. I occasionally stick my old bones on one and attempt to defy gravity.
