What sport(s) should my 6-yr-old son play?

Does he have a preference? Tennis is pretty athletic, maybe you're just playing inside your comfort zone.

I like martial arts for a lot of reasons, but it's really hard to find a good teacher, there are a lot of BS artists. the more they're into belts and trophies, the less good IMHO. I also think it requires a degree of mental maturity, maybe 6 is too young. I share your indifference to and worries about football, which strikes me as a commercialized tribal mating ceremony.

The obvious answer is probably the correct one.

If he wants to play a sport, he should play whichever sport he has fun with and actually wants to play.

Hockey to toughen him up and get a great social experience.

I am Canadian BTW