@JackieG
Dogs don't really do things because they're resentful. We could speculate endlessly on why he's decided to mark this area but the solution is still the same. He has to be managed as though he's not fully housetrained. Don't allow him to roam freely. If you can't actively supervise, crate him. Take him out to eliminate and reward him with a treat and praise him when he pees outside. Be consistent with the treats and praise. Don't scold or punish if he marks indoors.
In addition, neutering is supposed to help with marking behaviors. If he hasn't been, that might help.