I think I would take him to the vet, if nothing has changed at all in your home and he just started doing this. Were any of the other dogs puppies when he came to live with you, and are now maturing? Any new people in the house? Or any new furniture? Males who are not neutered can be VERY territorial. One of my dogs was so territorial that he peed on someone's foot!
But, if nothing has changed, it could be a bladder infection or a urinary tract infection. Could even possibly be a prostate problem.