20 lbs is too much, really you want to shoot for about 10-20% of the child's weight. So in order not damage the growing skeleton I would shoot for 5-8 lbs max. I know this is tough but with good gear it's totally doable. Mostly you need to lighten your load to compensate for all his stuff you will be carting. My son has an rei kids pack that's pretty good. I think it's the comet.