Well, if you look in your school rulebook's definition of dangerous weapons, pepperspray or a stun gun probably isnt listed. Pepperspray is ideal, it doesnt look very gun like, and it's even accepted among liberals.
Another thing you can look into getting him is a cheap pen knife, looks like a pen and can write, but you pull the top off and it's a knife.
I know I'll probably get flamed at suggesting him to get his kid one of these, but having equal odds of coming out alive/not injured is good in my mind. It seems like many people here have CCWs, yet live in the boonies or there is otherwise no direct threat toward you at the moment. That's fine. But, in my mind, a person with a gang going after him deserves the means to equal the odds, legal or not.
Teach your kid to fight, too. Another thing is to teach him to stay low-key, being low-key will keep you out of trouble, too.
As far as telling the school, in my experience as a 16 year old, telling the school will not do jack s*it, but that's what they expect kids to do now. Just go tell a guidance counselor, and all your problems will go away!
Tell the cops about the gang, that'd help way more than the school. You should probably still tell the school anyway, just because. If your school has a school cop, tell him, too. Just try to get everyone to know that there's a gang.