I think I had some success with Jeyes Fluid but I was trying to protect the whole garden from poo and I think Silent Roar didn’t work on such a big area. I was thinking that you could put some where he enters the garden or sprays. His spraying must be the most upsetting smell for your cats.
Edit: I don’t have a cat.
Plus you could try a dish of Silent Roar on the exterior window sill where he’s getting in, if your cats don’t use it.