Cat won't use litter, I've tried EVERYTHING!

[mollyandstan]

My boyfriend and I adopted a four year old male cat from the SPCA two years ago as a companion cat for our older female cat. His personality is shy - the employee at the SPCA told us that he came from a hoarding situation and we took him home because he looked so sad and scared hiding underneath his cat bed. We had a few problems once we gave him some time to adjust to our apartment - he began peeing on the bath mat a couple of weeks after we brought him home. We started hanging the bathmat up and as long as it remained that way, he would use the litter box.

He has been fine up until about four or five months ago. Since then, he has taken up peeing and pooping on the floor in the bathroom and in our sun room. We took him to the vet, and she checked him out and told us that there is nothing wrong with him, it's completely behavioural. She recommended moving the litter boxes to the spots he pees and poops on, and getting a third box for the spot in the sun room. We did this and it worked for a brief period. Soon, he was back to peeing and pooping on the floor.

We're pretty much at our wits end. We keep the litter clean, we've tried changing types of litters, I've used tinfoil in spots where he goes outside the litter boxes, then he just goes beside the tinfoil. We haven't moved residence, his relationship with the older cat has stayed the same, furniture hasn't been moved. I did recently move the bathroom litter boxes to the sun room and placed one box in each place where he pees/poops but he is still going outside of the box. One of the boxes is much larger than the others so he has lots of room to move around in it but he still won't use it. Our last resort would be to keep him in our bedroom for a couple of days with food and a litter box to try and retrain him but I would feel so guilty keeping him confined to one room - our room isn't very big to begin with and I don't want to risk him beginning to use our bed or laundry basket as a bathroom.

Fortunately, our other cat is very forgiving and adjusts to change well - I could put the litter boxes on top of the fridge and she would find it and use it without incident.

Should we just give up and buy a puppy training pad for the sun room? He'd probably still just poop/pee on the floor beside it. I need some ideas and advice!