REI has good ultralight backpacks for like $50 on sale.
The kids pack is 2lb, 4oz. https://www.rei.com/product/870881/rei-passage-38-pack-kids "I bought this for my 8 year old daughter's 1st backpacking trip."
And that's more than twice as heavy as his pack.
PS: Sure on it's own saving 1lb is not that big a deal. But, if you focus on saving a few ounces here and there it makes a huge difference.