Are you correcting him when he does it, or just running him outside? I'd correct him and let him know that is unacceptable...at least you catch him in the act! He may be trying to assert his 'dominance' in his goofy growing puppy brain and his new marking behavior is getting out of hand!
I think after a correction or two, he'll understand he should not be doing it. And I'd keep a good eye on him to make sure he's not doing it elsewhere in the house.