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My son became quite self conscious of his body around this age. It was unusual because I thought the whole body image thing was more a girl problem, but apparently it is a boy thing too. I was concerned about the impact on my son as he grew so I actually got him into running and swimming rather then weights. You’ll find running and swimming are all round toners for your body giving everything a nice shape, and also have several other benefits such as cardio improvements and general fitness and wellbeing.
Boys should not be lifting heavy weights until they are fully grown else they can tear their muscles when they go through growth spurts. That said, however, they can do some light training and just keep in shape and keep his muscles flexible.
It is definitely not safe to do the whole Arnold Schwarzenegger thing but on a small scale it is ok. Ask your son why he is doing it, he may just want to be faster for the track team or more all around toned to impress a girl. If you find his reasons then perhaps you could talk to a personal trainer to see what would be more suitable.