When my son started preschool, I bought him a really nice preschooler backpack (it measured 11.5 x14). He used it for the two years of preschool and we started kindergarten with it, but we had to get a larger one to finish the year. He would bring home so much stuff, plus the lunch, snacks, extra sweaters, etc. that he would have trouble packing up in the afternoon. We found a quick Lands End replacement at Sears. For the price, it was a good backpack. Since then, we have gotten both a pottery barn and LL Bean backpack. I like both of them and they have withstood a lot of wear and tear. If you have either store near you, I would recommend having your son try them on and try opening and closing the zippers. More than a fashion accessory, it needs to function and hold up to every day use. If you spend more now, it will be a backpack your child can use for many years.
Good luck,
C.