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Lifting weights won’t hurt him. Most readiness programs start around 7th grade and honestly kids can start sooner. The big thing is lifting with correct form. There are a ton of dumbbell excercise he can do that are safe and there is a lot of variety.

Body weight things like lunges. Lots and lots of lunges will make him faster and more explosive. It will also protect his hips and knees. Front, back and side lunges. He can do those with dumbbells or just do lots of them. Pull-ups are some of the best excersises there are along with dips and burpees.

At that age (even in HS) a lot of kids running forms are jacked up. He’ll get faster due to age but proper running form will make a big difference. There are a lot of good resources on developing good form and you don’t need much space.

Playing sports as long as he wants and he can drive him around will be great for him. It helps prevent injury from overuse (in the sense he is having his body do the same thing over and over). Kids that specialize tend to have more injuries.