I looked at your previous questions to figure out the age of your son. If I read correctly he is 13 (possibly 14), then your husband is right. Time to butt out mama. Having worked as a middle school teacher, I can tell you that your interference is going to make it worse for him. You are giving the bullies more ammo.
I have seen this a million times. A nice kid, for whatever reason, becomes the target. I imagine it's heartbreaking for your whole family. I understand that you are trying to make things better, but what you are doing is not working.
Kids who act like this, for the most part, do it because they can. From my observation, they seem to fall into two categories - master manipulators with parents who think they can do no wrong OR tragic circumstances and needing an outlet for their frustrations. It sounds like you have discovered one of each scenario. Nothing you do on your end will fix it or them.
It's time to give your son the tools he needs. First, get off social media. Close down the account completely. If you choose to open a new account for family and REAL friends, do so under a pseudonym with no pictures. Change his phone number. Again, only give it to family and tried and true friends. If the bullies loose their main source of harassment, they will get bored. He also needs to learn the fine line of balancing between standing up for himself and not engaging them. This is tricky. Face to face, yes argue it out. Texting, online...ignore. It's only adding fuel to the fire.
I am not a proponent of just "getting physical" over something because "boys will be boys". Now if something is started and you have to defend yourself, by all mean defend away. If you feel threatened or in danger, absolutely defend yourself. But if you choose to give your son the advice to beat this kid up, he will have to suffer the consequences of that choice. Those consequences could be a huge variety of things. Maybe it ends the bullying. Maybe your son gets hit butt kicked. Maybe he gets suspended. Maybe someone seriously gets injured. Maybe he gets arrested or ticketed for fighting on campus. Just some food for thought.
Stop mediating momma. Your intentions are good. If you are constantly stepping in and playing mediator, he will NEVER be able to stand on his own. This is a battle he must fight alone. You are trying to fight for him, which is making the target on his back bigger. It's all about roots and wings. You have given him good roots, now he needs to fly on his own. I know it's hard. It's heartbreaking. But parenting is never easy.