When I had my first job, my mom taught me to put at least 10% into savings, at least 10% into the church offering plate, and then I could do what I wanted with the rest. I usually saved a lot more than that, because I saw how fast it would add up if I kept adding to it. On a guess, I'd say I probably spent 20-30% of each check.
You didn't say how old he is. If this is the first time he's working, and he's 12 or 13, it might be a good time to let him learn how it works on his own, with a little guidance.
Now, that said, I'd also say that since you've *already* put your foot down, you should stick with it.
Another thing, that I just thought of, that my mom used to have us do was to wait 24 hours before purchasing something, to stave off impulse buys. If we still wanted it after 24 hours, then we could get it.