15 body weight squats, 1 to 5 pushups, 0 chinups, just get them to hang or pull if they can , 0 to 5 sit ups.
This is the absolute most I would ask of a fit untrained kid for the first time.
We all vary.
Up it as you see fit.
He is just a kid and is in no way conditioned to lift weights or the like.
At his age, form is key.
20kg is enough for anyone to make gains.
I can promise you he ain't 1 arm curling, pressing or rowing 20 kg for reps.
Teach him that we don't ego lift, we lift to make gains with good form and at his age the weights should be light.
This is only general advice and is in no way a plan for your son.