My male cat Neko (who is neutered) will holler if he smells another cat who has sprayed. We visited my mother once, and he found a place where my sister's cat had sprayed in her house (she didn't know it). He caterwauled the rest of the time we were at her house and for a couple of days after we got home.

Shortly after we moved last year, Neko got out of the apartment and into a common hallway area where a neighbor's cat, who allowed to wander outside during the day, had sprayed. We scooped Neko up and brought him back in, but he hollered again for a couple of days after that. He would also get out of sorts whenever that cat felt the need to mark the hallway (thanks neighbor!) and holler for a couple of days.

Chances are, even if your cat is neutered, his friend outside is not. That may be why he's howling -- it's his natural reaction to cat smells called pheromones. Mruffruff gives good advice for how to get a feral cat to warm up to you. Most of all, be patient. And when you earn his trust, get him vetted. You don't want him getting your cat sick with something.

It's good that your parents are willing to invest in this health and take in another cat as well. Good luck and keep us posted.