He could do some explosive medball work as well as medball circuit stuff. Would box jumps or uphill plyo stuff be possible? For strength work he could do a lot of single leg stuff where you don’t need much resistance other than body weight, also glute hamstring raises, barbell thrusters and any other strength exercises that don’t load up the spine as much. General Strength circuits and maybe some light lifting circuits with and without machines could be an option too. Wouldn’t help a ton with strength but could help some unless this kid is in great shape.
