This will be long, but I felt the need. The most important thing of all is to make it fun--games of any sort, anything like that. Just make it fun for him, so he WANTS to keep training. According to Kurz in "Science of Sports Training" the USSR had a specific system implemented to address the needs of athletes by age (both absolute and training age). Children in the first phase (2-3 years, or until 12 years old) were trained almost exclusively with "fun and games"--his words, not mine. The most important things they learned were tumbling (somersaults, rolls, break falls, etc) and running and jumping and throwing and agility games. The USSR coaches only used 5-10% of the total training time with sport specific drills, and under no circumstances whatever (not even bicycling sports) did the coaches put the children under a "heavy" training load. Total training hours in a whole year were only a maximum of 250. Variety of activity and motor patterns was insisted upon at ALL costs, even "sport specific" training.
According to Kurz and the Soviets' extensive studies, as well as several motor learning theories, there is very little to NO correlation between specificity of training and athletic ability. What he means by this is that the strongest kid was also generally the most agile, the speediest, and the most coordinated. ---> Contrast this situation with the one that we are familiar to in our adult world--the person trained to be strong is strong but not necessarily agile. In other words, there is a almost universal carryover from training of any sort in a child to his athletic ability. This is kind of what happens with beginner weightlifters--they make good progress on any kind of exercise program. It is only later that they have to specialize in strength or hypertrophy or fat loss to see results. Initially everything happens at once--just like in children.
So, in essence, make him run, jump, dodge, duck, roll, tumble, throw--get as many different movement patterns and coordination patterns and muscles involved as possible WITHOUT regard to any sort of specific strength program. Variety and fun are the absolute kings between the ages of 10-12. Kids learn different movements amazingly fast at this age, so expand his repetoire as much as possible. Besides, you can use these times as GPP for your own training :).
That doesn't mean don't lift, that just means don't lift heavy at all, and don't do the same exercises ever in subsequent weeks. You should focus on playing games with him. In addition to games and tumbling and running and all that, my suggestion would be to add in some sort of mini strongman thing going--good GPP for you both, it gives him a sense of working his muscles, and it gives him more variable movements. Toss something, drag something, sprint, toss something backwards over your head, whatever. But never do the same things two times in a row, or two weeks in a row. Maybe just 1 "strongman" day a week and 1 "weightlifting" day a week, with the other 2 days being games and sprints and agility and tumbling--you should do this with him, as it would give him someone to "beat" and anyway he'll have more fun if you involve yourself. Weights--stick with unilateral movements and/or dumbells to give his supporting muscles the most work. Exceptions I think would be light squatting if you can give him the right form early on.
Lastly, I recommend getting Kurz's "science of sports training" book and reading it--you'll learn more than you thought possible, even if some items are outdated by modern research. Hope this helps.