Wasn't his mother a widow?
Nope. That was Jeff who lived with his mother and grandfather. Jeff was replaced by Timmy when Tommy Reddig, the actor who played Jeff started growing a visible mustache; at the same time, George Cleveland who played the grandfather, died, and Jan Clayton, who played the mother, wanted out. Enter Timmy (Jon Provost) and his adoptive parents (Chloris Leachman and Jon Shepodd -- to be replaced rather quickly by June Lockhart and Hugh Reilly). I always thought Timmy was kind of a snot and should have been left in the well.