That is awful!

I tend to avoid conflict but as you have to to ride that bus i would suggest 2 things:
If they say something i would turn round and just stare at one or more of them - obviously only if you feel safe to do so.

Or talk to the bus driver about this 'abuse'. But as you said it could make things worse.

i think your explaination was good. I would refain from why he seems a good target incase of your DS starting to not be himself. 5 is way to young to have to deal with this and IMO the less he knows the better smile

The rudeness of some kids is unbelievable!! I would be mortified if one of those 11 year olds was mine!