Sorry for the lack of information. His enviroment has not been changed. Hope that is enough information for you. Sue, I am not surprised to learn that your dog is intact.
Intact males mark.
It's like their job. Some start early, as puppies, and some wait until much later to develop this lovely habit. An intact male that never marks from the day he was born until the day he dies is extremely rare. First, I would have him checked for medical issues, such as urinary tract or bladder infection. You need to rule that out first.