It could be a behaviour thing.
Make sure your dog has lots to do when no one is home.
Make sure he has lots of toys, keep the radio on etc. If at all possible try to get a neighbor to give a walk during the day.
At the very least, lavish tremendous praise on him the moment he pees during his regular walks.
He may then associate urination with praise which will hopefully make him hold it in so he can please you.