Arbitrary numbers (like 30% of his body weight) are interesting starting points for discussion, but the real determination is how much he can carry on the practice hikes.
It is even possible to be less than the “Low End Weight”, which is what you should be striving to make.
Clothing low-6.5 high-9.5 Personal Gear low-9.9 high-17.5 Tent Mate Gear low-3.1 high-4.1 Troop Gear (12 hikers) low-3.1 high-3.1 Optional Items low-0 high-6.0 Total Pack Weight low-22.6 pounds high-40.2 pounds (this is crazy high!
Down bags are not good because they are ineffective when wet.
The sleeping bag should have a comfort rating from 20° to 30° and should weigh no more than 3.5 pounds, but preferably less.
Personal First Aid Kit: The Troop gear includes a fully stocked first aid kit, but you should also carry a personal first aid kit to handle minor problems.