The rule of thumb I think is often used erroneously is 25% of the body weight.
My son is 5'3 and weighs 97 pounds. This summer he carried 28-30 pounds without complaint. I'm 6'0, 187 pounds. For long distance, my comfort level ends around 35 pounds, but under 30 pounds is much more comfortable. But I ended up carrying around 43 pounds. It was too much for both of us.
The danger with using a percentage body weight is you son will end up with a 45 pound pack and you will end up with a 27 pound pack if the percentage is 25%. It will be too much.
Personally, and this is a strictly PERSONAL opinion, is I feel each should carry you own weight. A small person's clothes are lighter, they need less food, and their sleeping bag can be lighter if they buy a small size. If each carries their own weight, then they will be more strict on how much they carry.
Of course some things can be shared, and it's natural to have a little imbalance with the bigger person carrying more.
Hope this somewhat of a non-answer helps. I'll be interested to see differing opinions.
Edited by Gershon (12/07/11 02:30 PM)