Every kid is different. I think a 10 year old has lots of energy and could hike many miles with a light pack, BUT, would get bored to tears. Hiking is pretty boring, compared to competitive sports or a video game. Kids usually do not get into appreciation of hiking until late teens.

You want your kid to continue to backpack so you MUST make it fun at first. How about involving your kid in the planning. Do some day hikes with the pack on so he gets a feel of the miles. Then let HIM decide. This would also start to teach him to read maps, some math skills to calculate rate of travel x miles = time on the trail. Do some car camping - time your set up and take down times for camp. Do you need to plan 2 hours from getting up to getting going? Or three.

In general I would not make a kid hike more than 4 hours then spend the next 4-6 having fun - fishing, exploring, making fires, games, whatever.

And if you are considering elevation gain, go out and try a bit first.