I would say (1) don’t complete his sentences and (2) figure out a way to let him vent when he’s had a bad speaking day - but follow his lead. My parents never acknowledged my stutter and to this day I harbor substantial resentment and I’m over 50. Others have dealt with this affliction better than me so maybe I’m not good for advice but it’s likely he is knotted up inside over it to a greater or lesser extent. Maybe therapy or support groups will suit him, maybe not. Knowing he Has at least 1 person he doesn’t have to “pass as fluent” with should be helpful for him. Also, maybe be on the lookout for avoidant behavior developing and try to facilitate him getting out with others. I am working now on trying to get “out there” more but it’s tough when you get this old. having a handicap like this tends to produce strong feelings of humiliation and frustration.