Neutered male spraying and now yowling

I have a 7 yr old male that was neutered at a young age. He has begun spraying in some places, especially the sides of plastic storage storage boxes or trash cans. Now he has also started yowling at night, and aggravating two of our other cats (spayed females) by stalking them. I've taken him to the vet, they could find nothing physically wrong. He's acting like an unfixed male, but why? I tried Feliway diffusers for a month, there was no change except they made my chihuahua start peeing in the house....so those had to go.
Any other solutions you could suggest?

Have you had a vet look at him? 

Yes. The vet didn't find anything physically wrong. I have been reading the threads about Prozac helping the spraying issue, so I will discuss that with the vet.

I know someone who had a neutered male cat with this exact situation. She tried everything you've mentioned--multiple vet visits, Feliway, Prozac--and nothing made a dent in his behaviors. 

His spraying had become so bad that she was on the verge of having him put down. He'd been an indoor cat with a bonded companion cat all of his life, but out of desperation she started letting him spend part of his time outdoors and that solved the problem.

Apparently he needed the great outdoors to work off some of that male steam. 

is there possibly a stray cat outside he is seeing? This will sometimes cause this behavior to start out of the blue. has there been some change in the household?

I did hear of a case where an altered male started acting like an intact male.  Turned out some tissue was missed during the neutering and it was just enough to pump out troublesome hormones.

Thank you for the responses! Outdoors is not an option, we live in a subdivision with a small yard and fairly busy street. I've been feeding a couple of ferals outside recently that I plan to trap and take to a rescue. He could possibly be smelling those, but the spraying behavior started prior to them hanging around.
I HAD wondered if they could have missed something when he was neutered. Didn't know this has actually happened to others. I will definitely bring this up at the next visit (Not the same vet who fixed him, we moved 2 years ago).

Hi-

I too have the same spraying problem. I have tried everything. I have 4 cats and only one cat, Max, has an issue. He is neutered. I got him and another male cat at the same time, when we lived in an apartment. When we moved into the house we adopted a female cat right after. About a year later after getting her, the problem started. This is about the same time that we started to see the the neighbors cats.   My neighbor does have a collection of Feral cats that sometimes come across the street, so we've been trying to keep up with scaring them away. In the past, one cat has come right up to the windows and harasses my cats. She even climbed the screens. I haven't seen her in over a year, so I don't feel the need to call the authorities but Max still sprays. The windows, the door, the kitchen counter, the walls, the furniture, my leg once, the fridge, cloths, the bed, the kitchen aid mixer- everything. Recently we rescued our 4th cat, 3rd male (we found him running across a busy street. He then jumped in a pond). Max seems to really like the kitten and has no change in his spraying habit. He is friendly to the other cats in the house and they all play nicely together as long as one cat doesn't continue to play when the other is done.

I am trying the feliway now- and trying to keep up with it. I have 4 around the house. 1 upstairs, 2 on the main floor, and 1 in the basement. I can tell that he is a little more clam... and snuggley.

I try to give Max more attention and take him on the leash outside when it is nice. He hangs out with us in the cat tent and the garden when it is nice. I live on a busy street so being an out side cat is not an option for us. I have tried startling him and making loud noises when i catch him spraying, I've also tried locking him in the bathroom with a clean litter box for a time out.

Is there anything I can do? I've talked to the vet multiple times- their answer is always Feliway. Please help.

Thank you!