re: 11 yo son started lifting in gym.

i did not start that young (probably 15 yo) but did the same split over summers in prep for football season. i was average size, strength ~5'8" 165lbs but fairly coordinated. my regrets from my experience are

1 - not focusing on the on the full body compound lifts - squat, dead, shoulder press, power clean. i knew about these at the time but did not like them because they were hard, and i was not a strong -willed kid. i did the easy body-build split your son is using

2 - not knowing about and practicing the knees over toes-type movements such as split squats (both weighted and non), cossack squats,backwards drag, tib raises, flexed calf raises. i believe these keep the lower leg, knees and hips strong in extreme flexion

3 - not knowing about and using the kettlebell swing; not doing sprints every day. trains power, which i needed

4 - not drinking enough water and minerals

just my 2c. best of luck