Our current foster is an intact male who has started marking from the smell. He’s a bit skittish but in general appears to be relaxed. Eating and using his litter box without issues. Any advice to mitigate this issue?
Won't the organization you're fostering for have him neutered? It really should be done asap, as once spraying becomes a habit it may be impossible to break.
I adopted a neighborhood stray when he was a few years old. Had him neutered right away, but 15 years later he's still marking stuff. He's always had to be an outdoor cat because of this.