Distant future is fine. Where ever you are is a good place to start. If he can keep adding a few seconds or minutes each day or week, he'll get there, even it takes a long time. Worst case scenario is he gets more fit. I have no idea if his worsening tremors could be related to cycling/not cycling.

When I cycle inside, I both watch TV and listen to music so there is always a beat. Oddly enough (for a 66 year old woman) I can pedal most rapidly when I'm watching sports. Go figure.

Inspiring story: I met a guy last year who could barely walk. At age 81, he was over the limit for PFP but he begged to get in the program and his doctor finally said okay. I saw him again last week and he walked over to me, albeit slowly, and announced with a big grin that he had gone skiing the week before. Not far, no moguls, but he had gone skiing!