My son is about the same. He calls me (scream) in the middle of the night and I have to go with him. I'll be installing motion sensor in the hallway/bathroom and also added a switch at his headboard so whenever he leaves the bathroom, it won't be dark 'til he reaches his bed.
Do you keep the bathroom light on at night? Another thing that might be unnerving him besides the darkness is the quiet. You might try having a radio softly playing in there.
If he was 4, I'd put a potty chair in his room just as a pragmatic issue (i.e., to avoid having to clean up all the time), and would expect he would outgrow it. But at 11, hmm, if the light and the music don't help, I guess I'd take this seriously as an anxiety issue and consult with a kids' therapist on the question.
How about a really good high beam flashlight on his nightstand that he could use?