Judo! and some boxing... your kid will be un-fuck-with-able.
Think about it... throw your ass while you eat these punches on the way down.
If you can find a Judo school that is traditional (Kodokan) it focuses more on self defense, whereas a lot of Judo schools focus on sport judo (olympic) but still be bad ass.
If you can find a Kyokushin Karate school (GSP trains in this I believe, pretty much MMA-style Karate)... also dope, probably the only form of Karate I would ever consider taking.
Then there's always Ju Jitsu (Japanese ) or Brazilian Jiu Jitsu (which split off from Judo a long time ago and focuses mostly on the ground)
Muay Thai as seen, is pretty damn effective, when taught by a legit instructor.
If you have no other choice but Tae Kwon Do.. it's a good base for teaching your son how to kick real good, but I would then move away from that into something like Muay Thai or Kyokushin Karate.
I myself will be getting back into Judo/Jiu Jitsu within the next week with some Boxing and start competing by the end of this year... need to start exercising again.