It must be a real pain in the bum, there is no worse smell than cat spray. Well not to me anyhow. Sorry if I'm suggesting things you've already tried, but can you chase other cats away when you see them so that he feels more confident in the garden? I know you can't spend your life running in and out, but is there something you could rig up with a sensor that would either spray other cats with water or make a loud (ish) noise to scare them away, but that you can turn off when your cats are in the garden (and not annoy the neighbours lol)? If the other cats that bully your tom belong to neighbours, is there any chance of talking to them to arrange for your cats to be allowed out at different times?
I also read somewhere that if cats are fouling the same places all the time, putting their food there should help because they don't like toileting (or spraying) where they eat.
I'm probably clutching at straws a bit, so hope someone can come up with somethingt useful. Best of luck.