your son sure is lucky! I think his height would help.

Are you American? REI carries some travel packs and backpacks designed for kids.

I have backpacked with a 9 year old boy, and while I don’t think my son weighed 100 lbs. (he was always exactly average for his age in weight and 90th percentile in height), the amount of stuff he could carry on a hot day, up and down steps, etc. was pretty limited. We ultimately limited what he carried to basically what you’d need for about a two day trek — two changes of clothes, toiletries, water (lots of) and snacks for the day, jacket and his miscellaneous stuff. His school pack (a good sized jansport) turned out to be about right for size. He could handle much more for short periods of time or if it was cool and level, but heat and uphill or steps really cut down on what he could comfortably carry at a reasonable pace for any distance. Train to subway to hotel is usually lots of walking and SEVERAL flights of stairs. We did lots of walking in preparation but should have done more "training" on steps and hills.

I thought I would write a quick update. Yes I am in the US and I went to REI yesterday. I found a nice internal Frame pack manufactured for REI for my som Caleb. It was very adjustable and the right size. We put 15 lbs in it and had him wear it for about 40 min in the store. I was also able to get a new pair of boots.