I used to hike frequently with mine, and considered getting them backpacks so they could haul some of their own water. However, when I did research, the recommendation was not to give them more than 10% of their weight to carry, which would be less than 2 lbs each for my girls. I decided I could carry that 3.5 pounds of water myself quite easily.
So I don't have any recommendations, but if all he is carrying is mail, I think any of them would be fine.
