That is a very vague question as I need more information about your son as the thesaurus defines a gym as an exercise room, fitness centre, sports centre or a physical education centre. I will assume that you are referring to strength training where resistance training with weights is performed to improve his physical abilities. I will also assume that he is younger than 14 years of age. Nevertheless, I recommend that whatever age he is it is important that you consult a doctor prior to introducing him to resistance training. The area of most concern in children is the potential for damage to the epiphysis (growth plates) of the long bones because cartilage is not as strong as bone. Children can benefit from a properly designed and supervised AGE appropriate weight training program. Often injury is due to being physically unprepared to participate. Correct training programs may limit injuries by preparing the body for the challenges of competition. In most cases it is the key to preventing injury.