The Invisible Man's Mother

Mrs. Zimmer welcomes a new neighbor for coffee, and the visitor can't stop bragging about her son, a very successful doctor. When the talk turns to Mrs. Zimmer's own son, she reluctantly admits he still lives at home and was born invisible, and the neighbor soon finds herself greeting an empty room as doors open and objects move on their own. Once the guest flees, Mrs. Zimmer turns on Arnold with a full round of maternal guilt about her humiliation, nagging an empty chair about his posture and his feet on the coffee table, before complaining to her husband that two perfectly visible parents somehow ended up with a son nobody can see.

TheTVDB and IMDb place this sketch in season 6, episode 13 (December 16, 1972), the episode with guests Anthony Newley and Bernadette Peters; the official YouTube clip's description labels it season 6, episode 4.