Ok. Well, I wouldn't really worry about him marking in Pet Smart. It would be hard for any non-neutered boy to resist! What with all of those other dogs around there.
I would say that the neutering will help, but...well... I don't know. I would do it ASAP. My kitty wasn't neutered until he was an adult, and I tell you, he still marks like a tom cat. The other two were neutered as little kittens, and they never mark. So, it might just be anecdotal, but I really believe that the earlier you neuter the better with a male animal.