I have had two lovely Burmese boys just over 12 months now. They were originally inside for 6 months then went outside, but now because of where I live must stay inside. I do have a small outside pen, but can't leave them in there because of the rules in our complex. Anyway, one of them in particular has started peeing inside on the floor every single day now for over a month. I think he is stressed, but so am I. I'm close to letting them both go to a new home. I'm forever cleaning up. Please help.
Since your cat is use to be outdoors, now he's an indoor cat, maybe the litter box confuses your pet? Wild guess. Call your vet and ask for suggestions. :)