Cat won't go outside

We have recently acquired a 7 year old cat. He has access to the outside and his food is outside and for the first three months we had him he went outside to go to the bathroom. But now he seems increasingly reluctant to go outside at all and has taken to peeing and pooping on the bathroom rug. How can we discourage him?

I have an inside outside cat as well. She uses the doggie door to get in and out. Ive put a litter box in the house, even though I know that she has access to the outside world. She goes through phases, some weeks we hardly see her at all and others, shes a complete homebody.
   
So maybe keeping a litter box inside the house as well and leaving your kitty access to the outside will help the situation. Is your kitty only going to the bathroom in that one area, or are there other areas in the house that he/she chooses?