What muscles do I stipulation to train...?

to be able to do that athletic move like a push up, but your legs are straight out?

Answers:

see the doctor

At what age and what other precautions should I use if I want my son to natural life weights?

Abs, triceps, chest, back, forearms, and adjectives round legs.

It's going to take a while tho. Doing those require a lotttt of strength everywhere. Protein shakes assistance.

Sex on the brain?

abs, chest, fore arms, triceps, biceps, thighs, back, of late about everything.