Renee, we had a similar problem with one of our cats - a female cat called Smudge! We already had 1 cat when we rescued Smudge and her sister at 5 weeks old, a year later we got another kitten making 4 cats. There was never any problems - then Tigger went missing leaving us with 3 cats, then we also got a dog. Smudge developed a problem with peeing in different places ALL over the house! I tried everything - one vet said it was because she needed hormone replacement as she obviously had an imbalance and had too much male hormone (?) Another just told me to get on with it, we had too many animals and she obviously didn't like it!!! We had her checked out and there was nothing physically wrong with her at all...it was a mental problem (another vet told us)
She actually peed on my husband when he was sleeping - can you imagine that!!!!!! Of course he wanted her put to sleep, in fact I think he may have been capable of doing it himself that night - and who could blame him! Another day she sauntered into the bathroom when he was sitting on the loo - lifted her tail, and peed down his leg - I kid you not
At a school reunion, a friend of my DH's who is a horse vet told us that it was because we had 3 cats, he asked us where she was in the pecking order and we admitted that she was probably the least dominant of the three. He said he had had a similar problem and it was his cats way of trying to get some dominance. Y'know, I think that was true - Smudge always ate last, she would stand back and let the other two eat first (even though she had her own dish) - and it wasn't until our old male cat died that her habits changed - she stopped peeing all over the place! - my DH was sooooooooo pleased LOL!!!
I know that this is no help to you - but maybe if he had a home where he was the only animal - and felt secure - then he would stop too! It may be that he is third in the pecking order of the cats in your house too and he is insecure because of his past and because he has been neutered ( I'm assuming so...) that this is his way of marking his territory...just a thought...
Fiona