Is there any way you can block off the area where he is peeing? I know you said when you put down a litter tray he just went beside it, but can you block off the general area for a while? I had a problem with Jaffa weeing in the hallway a couple of years ago and whatever caused it (I think it was caused by a combinatin of stress due to lots of workmen in my flat following a flood, tenison between him and my younger cat, and only one litter tray which is a covered one and I think he was feeling a bit vulnerable using that tray), I think it was habit as much as anything else that caused him to continue using that place. Although I cleaned thoroughly with a commercial stain/odour remover (Simple from pets at home) I was sure he was being attracted back to the same area by a lingering smell that I wasn't aware of. In addition to adding an open litter tray in the bathroom, I blocked off that part of the hall with some cardboard play boxes and he's never gone there since. Whatever the initial cause, cats sometimes return to the same place simply out of habit even when the initial cause has passed.