How old are you? Are YOU interested in getting into multi day backpacking or is that all your son's idea?
I would suggest letting him carry everything and you taking a very light pack.
But seriously - have a read of my most recent thread about Killing the Newbie, if you are middle aged, and not backpacked for a number of years. I'm assuming you haven't been doing it a lot if you are asking the questions. If your only concern is your son's growing body vs. a big heavy pack, I'd have him try on some packs full of stuff - water is a good choice since you can dump it out if it becomes uncomfortable - and go day hiking to see what's comfortable for him. I'd guess no more than 40 lbs (he's a big guy) - and since it's perfectly possible to be comfortable with a base weight of about 20 lbs before adding food and water, not sure why you'd have him carry more than 40.
Welcome to the wonderful world of "everything's relative" and "hike your own hike." Start slow and have a great time. It's perfectly acceptable to camp in the back yard with your gear to try it out before actually going overnight. I highly recommend going backpacking for a few miles and one night, then increasing miles done for one night trips, then two nights, then more... and not just picking a week and death-marching it. You will be a lot more comfortable that way.