Thanks everyone!
Yes, he is neutered - we fix all our cats at 6 months.
We've tried the Feliway - yes, it is expensive and no, it did not work
Looks like the next stop is the vet - I like the idea that maybe once the medications work we might be able to wean him off of it. I read somewhere...
We have four cats. Three males and one female. Everyone mostly gets along, but we've had MAJOR problems with spraying/marking since the newest one joined us a few summers ago. My son rescued this rascally little orange guy - he was so tiny and so sick that the vet didn't think he would make it...