Puddin' went to the groomer's today and returned with the verdict that he had FLEAS

. *I* didn't see any, he wasn't scratching, but they said he needed a dip.
So, if he has fleas, I need to do something for him. I've been trying to keep him as chemical-free as possible, so I'm really not sure what I/he should be using that's consistent with that goal.
Suggestions?