House Trained Dog Refusing to Go Outside to Pee?

I have a 4 year old mixed breed dog who is trained to go outside. Recently he has started refusing to go outside and now pees in the house. Sometimes he will go outside, but more often than not he won't. He refuses to go outside first thing in the morning; he just pees on the floor. I don't know how to get him to go outside. This has just recently become a problem.

By Michele R