Children are strange! LOL! Some indulge in the most peculiar behaviour but turn out just fine in the end. But if you are concerned, keep observing his communication and social skills.
GLM, that's so cool about how you handle this with your son. My mother loves to tell the story about how her younger sister (barely a year younger) bit her once when my mother was about 4. Their mom bit her sister back and she never did it again. My grandmother was one of the sweetest people you could ever meet, so I cannot even picture it, but it got my aunt's attention. My mother also said that when they were really naughty their mother would put a drop of tabasco on their tongue. I could never do that (do not want them to associate a food with discipline) although for my kids if I could somehow force them to eat a brussel sprout that would probably be a much scarier consequence to them. Good luck, Annig, and also to the rest of you moms (and dads!)