Getting him neutered should reduce this or maybe eliminate it, but neutered dogs sometimes mark, too. Riley was neutered at 6-months-old, and he marks things like crazy-outside, though. He's marked inside once. Well, it was twice-he marked two of my friends within a minute of each other on New Year's Eve. They have two dogs, including a Newf that I fostered, and I guess Riley was saying my friends were his.
At any rate, it should help to neuter him, but don't be surprised if you also have to work on some behavior modification as well.