You can give a gift certificate to a bookstore or a teacher's supply store. A gift card to a food place or beauty place is an ideal gift. If you want to give a personalized gift, help your son make a card that reads best teacher ever.
I would go with a gift card to Wal-Mart or Target... or whatever kind of store like that in your area. Soooo many teachers use their personal money to buy things for their classroom for the kids to enjoy. This way, they can either buy something that they need/want or use that gift card to buy more supplies instead of spending their own money.
My wife is a third grade teacher and I know the gifts she values the most that come from parents are, as kuriafiddie and nwarguy said, gift cards that allow her to buy things for the classroom. Another idea in addition to bookstores (always a winner) and big-box retail discount stores, are office supply stores (those are like a candy store for my wife). As for the student, hand-made cards and/or artwork are always a big hit.