I'm an external frame user, and I ended up buying internals for my two older kids (now 8 and 10) that backpack with me.

My kids are long and skinny - the 8 year old just went over 50 pounds, the 10 year old just went over 60 - 25th percentile for weight/75th percentile for height. I know what you're facing here...

topshot's advice about weight is spot on. Weight and warmth are the only important things here. Once you're walking, you're warm, but the bag has to be warm enought at night, because there might not be a fire. Weight is important because you can't move if it's too heavy.

I started all my kids (the third one is joining us this summer) with their ten essentials at 6, and I focused on weight when I bought them, and limited the amount of water they carried. I'm keeping the kid's packweight at 10% of body weight - and did three mile in/three mile out sort of hikes with them.

I just bought the almost 10 year old an REI comet - they are on sale at the outlet, and if you order them delivered to your local store they are easy to return if they don't fit. Internals are harder to load well than externals (as it sounds like you experienced).

An alternative is DJ2's pack which is a lightweight external that you'd have to build. I am still playing with this option for me. If I can get it to work this summer I'll start playing with it for kids.

Best advice (and it sounds like you're going there anyway) is to get a scale, make a gear list, and weigh everything that went into your son's pack last time. That will guide your efforts to lighten his load.

Second piece of advice; consider sharing a shelter. Scouts usually work on the buddy system, and tents are HEAVY unless you buy specialty gear (see the conversations on this site).

Good luck with the challenge you're facing. It's hard to teach new skills.

Steadman
(occassional backpacker and father of 4).