My first suggestion is to get him to the vet to find out if he has a bladder infection or is not well. That is one reason. The foster cat may have something that has been passed on that the place that sent him to you didn't pick up.

Another reason is that you have taken in another cat and then were taking a trip and leaving them alone. He was the only cat and you brought in another cat and it has upset his "territory" and is using his litter boxes. People believe it doesn't really matter, it does. After this I would not foster to see if that make a difference.

Go to Animal Planet, Jackson Galaxy and there is a cat Stella that was territorial and wanted attention amid all the other cats and was peeing. Check that out.

It might be that he is marking his territory since you got the other cat. But I agree with AsTimeGoesBy, definitely have the vet check him out to see if there is a physical reason if not then its probally psychological.

Thanks guys. He had just been to the vet 3 days before my trip. The vet had him on Covenia for a cold and checked his temperature out. There werent any other problems.
I do think its more a mental thing than a physical one. Or he just doesnt like the litter im using (Tidy Cats)

Did you change litter? Sometimes they don't like when you change the litter they are using. Each has a different smell to it. I use Tidy Cats, but when I used another litter, it left that perfume smell on my one cat's feet for a long time that day and that litter went out in the trash.

I try to stick to Tidy Cat but use Arm and Hammer sometimes. I spent extra time with my boy when i got home last night and for once, he didnt pee on the bed. Hope it was just a security thing and that he feels a little better now. We will see when i get home!!! =^..^=

You may be paying so much attention to the foster cat and your boy doesn't like it. So extra attention to your boy.

I think he just has abandonment issues even though my mom is with him all day. I loved on him and sat with him a while watching the leaves blow last night. He came to bed early and used the litter box and didnt touch the bed last night. Thank God! Hope it keeps up tonight! :)

It could be abandoment issues, once when my daughter came home for the weekend from college her cat pooped on the livingroom floor , especially since she brought a friend over with her. I guess our furfriends don't like sharing their time with us with another cat or human sometimes