i believe you have mistaken the sport of curling for the curling of weights, they are two completly different things. Just a thought. But as far as your question, I would say do cardio since a lot of football is running, as well as possibly sign up to a local gym where they might have a better slection of machines etc, plus possibly some actual classes for teens(with similar intrests.)

start with low wieghts, curlin, bench press, dead lift and squats. do high reps, low weight for toning and high weight low rep for muscle building. u could also try supplements but on the supplements contact a doc

Although your son is large for his age, most experts recommend against heavy weight training. His body, especially his bones are still developing.

I would consult his doctor first, then speak with a certified trainer.

Weight Lift at a certified gym I guess.