I don't know which illness he has but many chronic ones can take many years before they are a debilitating problem, other than flares ups like it sounds like yours has had this summer? I had a group T I loved with multiple sclerosis and it was long after the nearly 10 years I was in the group before she was unable to work anymore at all. Yes, she was out sick occasionally (and we had a second group leader so the group went on, though we did not talk about the other's illness) and sometimes for many weeks but there wasn't any need to immediately grieve? Maybe you can discuss someone you can see/talk to when/if your T thinks he's going to be out often in the future?