Hi Emily, this may not be much help, but can I suggest you get into the hospital bed yourself and see if there are any issues with its comfort? I say this because my mother complained bitterly about some discomfort in her hospital bed mattress...the nice ALS closet supplied us with a few different mattresses to try out, and we finally found one that worked. I thought she was just being a little princess. Sadly she passed away last month, and I actually climbed on the bed to rest while I was clearing out the apartment. I had a better idea of what she was experiencing stuck in that bed all the time. When the closet came to pick up the bed, I saw that there was actually a bar at the point where it bends, right under the butt area, when you raise the head-end of the bed. With the mattress cells running side to side across the bed, she'd actually wind up resting her butt on the bar, as it protruded side to side between the cells. It was excruciating to her, and I just sort of didn't understand it. If I had gotten in the bed when she started complaining, I would have understood it better. She wasn't able to articulate "why" it was so uncomfortable. In hindsight, if I had known, I would have ordered a bed with solid base sections, instead of the less expensive one which had a spring base.
I did get Hoyered a few times to see what that was like, and I know it wasn't too smart, but I tried the Bipap machine too for a couple of breaths (which was a shock)...but I think it might help to empathize a bit with our PALS by experiencing what they experience, when it comes to comfort.
I hope you can figure out what might be keeping him out of the bed, whether it's comfort, or just the idea of being in a hospital bed.