Dear Nikki
The important thing he must do is to try and make life for “Ollie†as normal as possible. If he poops in the house just calmly pick it up, pop a bit into the soil box and clean the area thoroughly with biological washing powder and then spray with an odour eliminator. Do not get cross. If he has started to prefer another substrate then use this to line the box and then gradually replace a preferred litter little by little over time. His environment has changed and so will his usual routine so try and make it as normal as possible. Perhaps a few chase games and some tasty treats might make him feel a bit better.
If he is still going in the wrong places I would suggest that he asks advice from a qualified animal behaviourist recommended by his vet.
Hope that helps a little
Rosie Barclay