How do i stop my kitten from pooing indoors and get him to go outside?

My kitten is 6 months old now. Ive had him since he was 10Wks. At first it was fine he always used the litter tray. Then he got locked in living room and ever since he started pooing behind sofas and in the kitchen. We banished him to the kitchen with his litter tray and he got the hang of it. We then moved it outside and for a couple of weeks he was fine. Then suddenly he's pooing in the house again. How do I stop it. I've tried everything.