When I hear of this type of situation, my first thought is that your great grandfather probably needs more skilled support, either to help him bathe or to re-teach him
bathing skills. Activities like showering are considered "activities of daily living," and an occupational therapist is very skilled at assessing and making a plan for how to carry out this task as independently and safely as possible. There could be a multitude of reasons why your great grandfather hasn't showered in a year and I encourage you to investigate this further. Start by discussing the situation with his doctor. I'd be interested in his cognitive functioning and ability to understand the "why" and "how" of the task. Does he have a diagnosis of dementia? This can very often inhibit one from carrying out hygiene tasks. An occupational therapist can help you look into the design and function of his bathroom. Sometimes too much clutter around the sink or bath make processing and organizing the steps needed to wash challenging. Does your great grandfather have any visual impairments, or tactile or temperature sensitivities that might prevent him from showering? Additionally, he may have had a negative experience with showering in the past that has created negative associations and resistance to the task, such as falling or getting burned from hot water.
As you can see, there are many things to consider. You need professional assistance to help you get to the bottom of the situation, and to come up with a solution that’s individualized for your dad. Be sure and talk to your great grandfather's doctor about having him and his bathroom evaluated by an occupational therapist. With help, your great grandfather may learn to enjoy and achieve bathing again, or at least to understand why it's necessary. Or, you'll learn he needs caregiver support for this. Either way, there's hope for a change.