What is his setup at your place like?

I've experienced intact tomcat wailing out their mating calls.  It is
very likely the cat is advertising his presence to any female within
earshot.

What can you tell from his behavior other than the noise? Does he go round
to doors and windows wanting out? Does he follow you around howling, and
jump in your lap whenever he can? Does he stop howling when you feed or
water him?

What was his former home like? Was he an outdoor cat, and are you keeping
him indoors? Did he have other cats or children in his former home? Was
there some other animal or human he was particularly fond of?

Has the vet heard the howling, does it sound to the vet like some sort of
pain?

Margaret

Don't rule out hyperthyroidism.  While rare in younger cats under 10 it does
cause howling.

Paul

Maybe he's part Siamese?  Mine yells his fool head off anytime he's caged. When
I pick him up at the vet's, I can hear the poor  thing yowling from the back
when I walk in.

Sherry

He is un-neutered and thus will howl quite a lot until he is. He is also
calling for attention from the humans as well. I think once you get him
fixed then his howling will reduce a little unless you just have one of
those very chatty cats.