SHAFAQNA – The Grand Ayatollah Sistani answered questions about respecting father and mother.
Question: What is the ruling about the son who does not greet (say Salaam to) his father? How about if the son greets the father but the father does not reply?
The Grand Ayatollah Sistani: Respecting father and mother is Wajib; even if they do not reply, you keep greeting them.
Source: PERSIAN SHAFAQNA