This is shocking and heartbreaking. Why do the other kids know that he wears a pullup in the first place? That should be confidential and it's up to the school to make sure that it remains that way as part of confidentiality around 504 plans. I would go to the school asap as others have said and put all of your concerns in writing demanding immediate action. If they don't then take it further asap, don't give them too much time to dither as this is very serious.
Your poor little guy. I hate that this is happening to him and at six years old it's outrageous. You can do some roll playing with him ie. what to say to the bullies but if they're actually violent I would not want to encourage them. I've not actually heard of physical violence at that age (I have a six and seven year old). I hope you get this sorted very soon.
