Dad sleeps all day, has no energy, some memory problems. Rapid decline over past 4 months. Checks out good physically. What is going on?

Back in January I posted about my now 92 year old Dad having a car accident with some confusion. He did have a UTI, and at first he improved with antibiotics. But he continues to be on a steep decline. He still has some confusion and memory issues that aren’t connected to a UTI but he scored 25 on the MMSE and his confusion usually clears up pretty fast. The most concerning thing is how much he sleeps. He goes to bed at 8 pm and sleeps until 9 am. But he then naps, or gets back in bed, for hours throughout the day. Only gets up to eat or go to the bathroom. Some days are better than others, but most days are like I described. All of his vital signs and blood work are great, so there is no obvious physical problem. And the physical therapist says he is very strong for his age. He just says he doesn’t feel good. He also has really let himself go from a hygiene perspective, shaving only once a week and rarely showering. It’s like he is too tired to do it. He was always a very active man and it is hard to see him wasting away like this. Any ideas what might be going on? I know depression is part of it, but could this be the start of the dying process?