Hahahaha!!! Oh my god, I cannot stop laughing! Is that young Jeffrey in the picture??? Lol :D
Ok, straight face and sensible answer... *ahem*
Whilst it's tempting to tell your kids to stand up for themselves and hit back, it will only turn them into a little thug - which is not something you want, I'm sure.
Why don't you get him into karate or something like that? It will give him some confidence and show him how to defend himself, but not attack.