Cat is driving my crazy!!!!!?

Ok... I have posted this question before and the solution worked for about a week.. Basically my cat.. Orion.. ever since I moved into my new house (and I have been there for about 6 months now) has decided that the carpet is a much better place to deficate then his box which is a mere 4 feet away... Now... I have two cats. The other cat is not a problem. But they were sharing a box. I thought maybe he didnt like sharing a box so I have gotten him a new box of his own and placed it in a slightly better location that I thought he would preferre. Well he seems happy for about a week and a half. Happily going in it with no problem. Now all of a sudden he has resumed his carpet pooing habit which is quite disgusting to say the least and I really dont know what else to do. I really cannot move the boxes in another part of the house as this part of the house is idea with it being a concrete floor. Any advice would be greatly appreciated as I am at my wits end!