Is a son considered undutiful if his parents are pleased with him, even though he commits sins against them that they are unaware of, such as backbiting, and is his sin limited to backbiting, or is he considered undutiful?

It is not permissible for a child to harm their parents, whether in word or deed. Rather, they must treat them kindly. If a child errs, then repents, and their parents forgive them, Allah will forgive them and be pleased with them, based on the hadith: "The pleasure of the Lord is in the pleasure of the father, and the displeasure of the Lord is in the displeasure of the father." As for backbiting (gheebah), it is a major sin, and it is even more prohibited when committed against parents. Whoever commits it must offer sincere repentance and seek their forgiveness. If they forgive them and are pleased with them, Allah will be pleased with them and forgive them.