There is nothing better than a father who is an experienced lifter teaching his son the "PROPER" way to lift and train, thats some priceless shit brother. I would probably start my son out when he is around 12 years old, prior to that I would encourage him to participate in whatever sports he seems to enjoy the most then apply the training at a moderate pace to aid him in his goals that he has for the particular sports he wants to participate in. Sticking to push ups, abdominal exercises, pullups and powerlifting movements would be a smart move at the beginning alway emphasizing safety. peace