I race a 750 race bike
it's hard to give a good opinion on riding position as I'm not sure if it work transfer over to a small bike... but... he is looking good, he should keep his body square to the bike, if his butt is off the bike so should his upper body, pretend that there are mirrors on the bike and he should "kiss the mirror"
LOOK thru the turns with his head, NOT with his eye balls! don't look so much at the rider in front of him, look past that rider, but use peripheral vision!
Good luck to your son and you!