Re: whats a good age for kids to start boxing?
anything under 12-14 years old you want them to get as much athletic variety as possible to develop body awareness, strong mental habits and avoid burnout. let him come to the gym and play boxing, keeping in mind the fundamentals of hands up and head down. make it fun with games for lateral, fwd, bkwd shuffles, jump rope, basic mitts, etc.
get him into sports like swimming (aerobic conditioning, flexibility, fun), track, soccer (foot eye co-ordination, fun) and gymnastics or some other tumbling activity like judo/jiu-jitsu (to have fun with combat arts where he will learn discipline and the concept of using his hips, and 'create space for offense and take space away for defense' which are both huge for advanced infighting strategies).
the biggest thing with kids that age are to let them have fun while they learn the absolute basic skills (don't even worry about strategies yet). if i were teaching basketball i would break it down to dribbling, passing, and shooting. for boxing i break it down to feet, hands, and head.
feet-jump rope, shuffles and squats side to side/fwd/bkwd,
head-take a towel and tie a knot in the end of it and slowly swing it at his head so he has to duck (straight down, not a roll, keeping it as simple as possible)
hands-mitts or intervals for 10 sec on and 10 sec off (this is more about having fun than conditioning -punch, punch, punch, keep goin, keep going!)
give a lot of positive reinforcement and encourage him to be a good little worker. in our society even at that age little people know that daddy (and all the other men in their lives) is a 'worker'. he wants to work and be rewarded like a big boy. play with this idea to encourage a strong work ethic early on. don't make it dreadful or burdensome - make work FUN!