I have 2 boy (one a man now) but still my baby
I kept them physical active they ran with me (first in a stroller) then along with their bikes. I started both of them off with body weight exercise. I would take them to the playground and run a "military like course" they had to climb, chin ups, jump over stuff ( when they were younger I supported them then as they grew they could swing like monkeys with good healthy upper body strength. The kids now just don't get the same activity.
My guys are 16 and 20 and still think that I was the most fun mom. They tell me all the kids thought I was the coolest
cause I actually played on the playground with them. I had all the kids wanting to run the course with my boys.
I can't remember at the time but the science suggest no weight lifting until after puberty. My guys started with weights around age 13. You may want to post in the Ask Cathe. She has sons around that age or early teen if I'm not mistaken.
There may also be a middle school or high school coach/pe teacher on the forum that would no for sure.
Its cool you are close to the nephew maybe one day he will do a cathe video with you