The Non-Stop Howling Cat Named Wheezy

Hi guys! My name is Matthew and I took in a stray cat my friend and I found about a year ago (named him Wheezy because he couldn't meow). He was our third cat, the only male. Our other cats are six and eight years old, both are female and both are spayed. When I brought the stray cat named Wheezy home, it was clear my cat's weren't happy. They hissed at him constantly and sometimes even fought. Wheezy never seemed to mind them from day one though, despite their hatred towards him. Wheezy is extremely friendly and never hisses at anything or anyone, even dogs. After a couple of days, the tension between them fizzled but something else happened.

One day I heard a low howl, something I had never heard come from a cat before. It wasn't frequent at all, in fact, it only happened once that day. As time went on however, the howling became more frequent and louder. He would howl all day and all night. I did some research and read that male cats often howl if they aren't neutered. Not being able to stand it anymore, I took him to the local vet for a check up. The vet said he was neutered, healthy, and based on the color and size of his teeth, about a year old. I assumed that maybe this poor cat had been abandon by their previous owner due to his relentless howling.

Having gotten extremely attached to this cat, I couldn't bring myself to let him go. It's been almost a year and he continues to howl and sometimes even hump people. It's becoming unbearable and I'm searching for a solution but can't find anything. Does anybody have any ideas? Should I take him back to the vet for a thorough examination? My parent's were never too keen on keeping him and this howling isn't helping. Thanks for any future input!

Did the vet check his blood for testosterone.  There is a possibility that he has a testicle that didn't descend and only one was removed when he was neutered.  Could even be possible that neither descended.  The only other physical reason I can think of is hyperthyroidism but he is really young for that.

Almost positive he didn't. So perhaps a second visit to the vet is the answer?

I have a howler, but he's neutered and doesn't howl all the time.  He only does it when he wants attention and at night when we first turn out all the lights and he's trying to settle down. 

I would take the cat to the vet and spring for a full blood panel and make sure the cats numbers are all in check and there are no B12 issues. Have them check and see if he really was fixed and he has no internal issues. Yowling can be many things but if the animal is not feeling well he can yowl so it is worth ruling out. Is the cat locked away from people when he yowls or does he just find a spot and go at it?