I'm betting it's because he's not neutered. Not all marking is in the form of vertical spraying - horizontal peeing is often used to claim territory too. He's probably got some hormones surging and doesn't really know what to do with them! I wouldn't worry about it too much for now.
Good luck with appointment on Friday. (they're not going to vaccinate him at the same time, are they?)
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler