It may become harder to correct these problems if left unchecked because it simply becomes habit. If the above suggestions fail to produce results your cat may benefit from confinement. The idea behind this is to find a room where the only desirable place to potty is the box. It is generally necessary to do this for a period of 7-14 days to reestablish normal potty habits. Possibly someone has already mentioned that if the cat doesn't shape up he will have to go.
Desperate situations call for desperate measures.