The best way to make sure you get the best prices on books is to tell your kid he has to pay for it himself, and arm him with a list of possible sites. Chances are, he might become a lot more interested in comparison shopping

Then if you want to pay for his books (which I see nothing wrong with), you can reimburse him for what he ordered.
There was a used textbook store near my campus, unaffiliated with the school, that had great prices on books. He may want to see if a store like that exists near his school. Also, he should email professors and ask if the earlier edition of any books are acceptable.