Gee, I wish you had asked this sooner . . .
I mean a few years ago.
First, THANK YOU for bringing up your grandson
in a stable, loving environment.
I'm sure your grandson is a lot better off
than he might have been in other circumstances.
Next, is there a family friend, perhaps an uncle or cousin,
from your side of the family OR from his mom's side of the family,
who could become a "big brother" kind of companion
on a regular (monthly? more frequently?) basis.
Not to teach him how to fight more effectively,
but to convey various "manly" attitudes and strength.
He will be stronger when he can incorporate the belief
that what stupid people say to him reflects on them,
not on him. He will have more tools to work with
if he can learn various self-esteem-building lessons.
Meanwhile, if there are any KARATE or TAE KWON DO
or KRAV MAGA classes available in your area,
it might be a good idea to enroll him in those lessons,
not simply for the physical ability but, more specifically,
for learning focus, drawing on mental strengths,
setting up a mindset that will help him protect himself
from stupidity and "the slings and arrows of outrageous fortune".
Good luck.
.