I would try to get him off the Royal Canin, I really think it might be the corn or other fillers that is causing the diarrhea. I still have another F2 here with runny stools so I've just switched to Earthborn to see if that will help. However, I have found that Iams Hairball has the highest fiber and that has worked well for me on a couple of cats who have had sensitive stomachs and are prone to diarrhea.
As for the inappropriate elimination - I'm not sure where that is coming from, he was always quite faithful with the litterbox here, and they were shared by several other kitties. You might want to try adding some Cat Attract to the litter to see if that fixes things. What kind of litter and litterbox are you using? I know we talked about this, but we were using wood pellets here, and he seemed to favor an open (uncovered) litterbox.
As for the inappropriate elimination - I'm not sure where that is coming from, he was always quite faithful with the litterbox here, and they were shared by several other kitties. You might want to try adding some Cat Attract to the litter to see if that fixes things. What kind of litter and litterbox are you using? I know we talked about this, but we were using wood pellets here, and he seemed to favor an open (uncovered) litterbox.