You guys might like the WC, who knows...but...internal could mean anything from hours of strenuous low stances, breathing and fa jing cultivation to meticulous repetion of technique and technical explorations without resistance from an opponent or punchbag.
...so, it could be really hardwork and great for tendons and stabilising muscle groups or it could be quite poor for the physiques development.
Based only on your description as "internal" I would say try Judo as it has a good child development programme built into its structure (thanks to governing bodies and olympic status) and the art itself has great exercises and techniques which promote the development of core muscles and tendons while not teaching eye gouging strikes and knee crushing low front kicks!
Judo may be percieved as a sport or an art that has been de-clawed but that is the ignorant view. You soon learn it has a very real, raw power after youve been smashed through the mat.
I think Brazilian Jiu Jitsu is a good form of exercise for the muscles and tendons too but its more expensive usually and not many children seem to take it in UK...if thats where u are from? ...yes, BJJ or Judo. Then he can add Karate or Ju Jitsu or Wing Chun or whatever later on when he is a bit older and really has caught the budo bug.
peace