I would agree that walking away isn't a good option...
There will always be bullys, no matter where you go. Always. Your son may as well learn to deal with them now, while you're waiting in the wings to pick him back up.
I only had to stand up to one kid and all the others left me alone... at the Boy's & Girls Club. He was pointing his finger at me and being really mean. I told him that if he didn't stop I would bend his finger backward. He didn't stop. I bent it backward 'till he hollered for mercy. It's a defining point in my childhood to this day. (not that I go bending people's fingers backward all the time) But there's something to be said for learning when to stand up for yourself and when it's not worth it. If he's not enjoying scouting, then it's not worth it - find a different interest and help him invest himself in that. Maybe it's not giving up, but changing? Re-evaluating.
If the boys are bigger than him, sure, get him karate classes - but he'll need to learn that it's for defense only. He can't be planning to bully them with his new skills.
Also, maybe he can get a couple friends together to stand up to the bully kid(s). A unified front can scare a bully into shape without any physical contact at all. They bully for the power - take the power away and they're usually done.
I hope all this helps you and that you find a happy solution!
T.