No idea about the immunotherapy part, but good hand hygiene is usually the way to go. In hospital environments, nurses and other people constantly come and go, and he's more likely to catch something there. It's how my wife first caught covid.

Just stay away from people who knowingly have things like colds, chickenpox, shingles etc, and it should be fine. When my wife's (she is much younger than your dad, plus her lungs weren't compromised) immune system was next to nothing, she caught shingles, but that was because the virus reactivated. Kids are usually the main source of infection, so if you have young kids, being more careful with them is probably a good idea. I know it's tough, but it's almost impossible to eliminate exposure to viruses etc unless you're in one of those sealed treatment rooms.

But yeah, good hand hygiene is more than half the battle. Just remember what you learned during covid, and that should be enough. All that said, his cancer team would tell you if you had to keep him and yourselves isolated. We were always told it was fine. She just got told to keep away from buses, and that was just after the UK came out of lockdown.