It is very dangerous and counterproductive for a child to weight train. For now, he can work on skills (speed, agility, ball handling, endurance, and game tactics & strategy. Just as a warning, about 90% of all kids who train to hard and or are pushed to hard by parents burn out before they get out of high school. For a six-yer-old, any sport should be about fun and friends. If he has a passion and talent, you'll have plenty of time to make him into the next superstar.