Help! Please!

Dr. Watson, my male Basset hound, will be 4 this December and still goes through periods wherein he uses the bathroom inside when left alone. There were long stretches during the South Carolina summer when I felt it was too hot to leave him outside during the workday so he stayed inside. Aside from a couple of instances he did great. For the past several days he’s used the bathroom inside again. Last night I removed his water bowl prior to bed. He went out once in the middle of the night which is his routine and again before I left for work. I came home to wet floors again. I’m at my wits end and know that there will be winter days that are too cold for him. Any help, suggestions, ideas, experiences are desperately needed and very welcome.