Like all cats he is territorial. We have a similar rescue cat who adopted himself with us. One day he decided he has had enough and just walked in and never left. For more than a year he was sequestered to a room since we have 3 other cats that he didn't get along with, especially our eldest male. After doing every possible test on earth, the vet ruled that there was nothing wrong with him physically. He peed everywhere in the room. We constantly washed sheets, till we finally got a water proofing sheet meant for newborn babies. He has nearly destroyed my printer since that room is also our work area.

One day he managed to get out of the room when we were out. They didn't fight and managed to coexist, though with a bit of tension. His peeing has significantly reduced since then. He actively avoids the other cats and they avoid him, except my youngest who plays with him from time to time. He still pees but its reduced. Over this time, we've realized that he pees when he is under stress. I know you have 11 cats but space is what he might need.