Is Manny neutered? From what I've read, there is a lot of research stating that it's a good thing to wait til a year before neutering, but he is getting close. Neutering may help. If you are concerned when going to people's homes, you could get him a belly band:
I like belly bands that are made of terry cloth or other thick material with padding. This is a short-term solution for visiting! I never leave a belly band on long-term if I'm not around to supervise. I have a Cavalier that I co-own and show who isn't neutered and he will occasionally mark in a new environment so I use a belly band on him.
Holly, Oliver, Rosalita, and Scarlett