Monchika- Aww how sad, i hope your DS is ok?
MY DS also does not retaliate when being bullied, this happens at childcare and unfortuantly he has now become a target for some of the bullies as they know he will not do anything to stop it/protect himself.
He does say "Stop!! ,I dont like it" "your making me sad,upset,hurt,angry"ect.

I am currently working with the director and his room leader to help implement new stratigies.

I too have thought about self defence classes.

Goodluck, let us know how it goes please