If he starts to jump the way my son does, you will want the stock springs. Our bike is still 3mm from making the 70mm sag recommendations. My son doesn't mind try setting the forks at 2 or 3 clicks out, and baseline the shock at 1 1/2 out. Put the fork height as low as you can to match the rear, but don't let it hit the bars. Then go out for a test and tune day.
During the tuning session, have him get warmed up good, and when he brings the bike back, take your screwdriver to the fork and shock. Make 1 click adjustments to the fork, or 1/4 turn adjustments to the shock. Sometimes change nothing, but never tell him what you did. Send him back out, and ask him if it was better or worse. You might have to inquire about which sections of the track it felt different on. Keep notes on your changes, and make note of his comments.
Also, watch him closely, If he seems to be jumping with the nose too high or too low, make your adjustments to help correct that. Nose high means soften the front or harden the rear.
Another thing you can do, especially if he's jumping, is put a zip tie on the fork tube (against the dust seal) and measure how far it gets pushed, this will tell you how much of the suspension travel he is using.
Once you feel confident that the suspension is as good as it will get, put him on the bike and let him ride. He will get used to the power eventually, and soon you will see him hanging the rear end out while ripping through the corners. You also might notice the occasional wheelie.