Yes, probably removing the cat tree completey for the time being and putting a litter tray in it's place would be the best idea.
I've found simple solution to be the best available ezyme cleaner for pets in the uk, I've had good results with another brand which had a much nicer scent but you'd have to mail-order that from the states. Buying simple solution from pets at home gets quite expensive, but if you only need it for a temporary situation with a pet then its not so bad.....You'd only save about a pound at a show so pets at home would be the best option for you really.
I do think it should be used in conjunction with other methods just to kinda hedge your bets - i.e moving trays around to problem areas, increasing number of trays, etc etc
Even try putting a small bit of litter with his scent on it in a clean tray to encourage him to use it!
On the subject of Feliway - Personally I don't rate it at all, although others I know use it to great effect. Maybe my cats just can't be fooled so easily, one girl even decided to spray the diffuser....I guess thats what she thought of the calming pheromones!
If you do plan to try the feliway then avoid buying from your local vetenary clinic, our vets wanted £36 for one diffuser.....I bought two off ebay for £32!