I have had similar experience in the recent past.
He was 80, and decorating the house/walking 5+ miles easily, drinking with his friends in the pub, going to football stadiums before he was ill.
Treating the main issue was tough (intensive care for 2 weeks plus weeks on the ward), but then the subsequent infections started. As someone said upthread, he was t able to fight off the minor infections because he was weak from the original illness, plus stuck in bed all day with no fresh air, plus eating minute amounts of rubbish food.
We got him home, but only by ensuring someone was with him for the majority of every day, taking in food, reading the paper with him, taking him for endless walks around the hospital (building up very gradually from a wheelchair trip to the cafe for lunch, and eventually to laps around the floor late over about 2 weeks).
We basically did respite care ourselves. The hospital don't have the staff or the space or the time for intensive respite care, so if you want it doing then IME you have to do it yourself!
Ask to speak to the Occupational Therapist and Physio and ask them to put together a plan that you can work on with him when you're visiting. They will absolutely understand that the hospital don't have the resource for respite and should be glad that you're willing to do things with him. If they won't do that, see if you can get a private OT in to see him to do the plan (OT more important than physio as long as spine healed enough to walk?). We were lucky that we have an OT in the family, so followed their instructions!
Good luck. Don't underestimate how important mental stimulus and regular movement are. It's very easy for the hospital to leave him parked in bed all day, plenty of people are quite happy to spend all day in bed "while they recover", and visitors are often happy to chat to a bed bound patient throughout their two hour visit. An 80 year old isn't going to get better by lying in bed; they're going to get worse and worse. Get him up, get him eating, get him reading/doing puzzles/calling friends.