I am the last one to give advice (considering MY problems), but it might have been the combination of Scout being a pest and the fact that these new walls an carpets no longer smell like him (where the old ones would have). AND I know the expense of feliway, but the spray type is our best bet for immediate results. The plug-ins take a while to "work". Spray the stuff right where he peed and do so daily for a month.
If the spraying persists you may need to confine him (like I did) to "retrain".
Since this is the fisrt and only spot perhaps you could place a covered litter box at this place (only temparary) so if he goes back to the same place a box will be there. (and I suggest covered because if he stands to spray it will hit the back of the box and not the wall).
Good luck, I know the frustration you are feeling.