Let him pack his own bag.
Mine went to rehab without any packing, not even a change of clothing.
They provided all the non-clothing essentials, shampoo, toothpaste, deodorant, razor, non-alcoholic mouthwash.
They also had laundry, everyone had to do their own (unless you're my XABF, then you sweet talk someone else to do it for you, after sweet-talking your daughter into buying you more clothing), detergent was provided.
Whether he packs or does not pack, he won't need anything there (aside from clothing, and if he doesn't pack that, oh well), so let him pack himself, and learn from anything he wanted to bring but didn't.