Mamma is a Man
The moment I saw the actor playing Dreyfus' mother, I said to myself 'I know that face,' and that it could belong (at that age) to a person of either sex. Coming to the cast list here at IMDb, the player's name listed is "Carlotta Gerson": you'll see that her resume is not especially revealing.
Then it struck me: it's Paul Mazursky, the director of the film! I don't know if it's an Alan Smithee kind of thing, or because there were scheduling or financial issues that arose at the last minute, but I'll eat my hat if that's not Paul playing the role.
(10 minutes later: oops, I've just read about Mazursky's role in the 'Trivia' section; well, no harm in repeating it, I hope.)