Does parents forgiving their son expiate his sins, or is repentance necessary along with forgiveness?

Quarrelling with parents is an act of disobedience (ʿuqūq) and a major sin (kabīrah), necessitating repentance (tawbah). Seeking forgiveness from parents is part of repentance, because a sin related to the right of a human being requires seeking their absolution. This does not waive God's right to repentance, which has its own conditions.