You did just what I would have done -- taking the loot back and having him apologize for what he'd done. If that didn't work, I have two ideas:
One -- have him talk to a police officer (a nice one -- the idea is to be turned off from taking things but NOT afraid of police) about stealing and what happens to people who steal.
Two -- let him experience what it's like on the other end. "Steal" something from him. Let him suffer from the loss for a while. Talk about how terrible it is to have something stolen. How the thief maybe didn't intend for him to suffer but that the thief DID do something wrong... Have him write a note and post it asking the thief to please return it. Then, return it while he's at school with an apology note attached -- "now I know stealing is wrong and I will never do it again." Do NOT let on that this was a lesson and that you were the thief!!! (You want him to learn about stealing not that "Mom can be sneaky"!)
Good luck. That your son has honest parents is the best thing for him! You set an example of how to behave and he will follow.