I'm going to stand up to my bully by using his mom's disease
Why would you bring him mother into it?
Because it's the only thing he understands.
That is distasteful, it's wrong to bring up his mother's disease to manipulate him. You should leave his mother out of it.
I don't care.
If you are having a problem with a bully then get an authority involved. As tempting as it is, it is not okay to use someone's loved ones disease against them.