The hajj of young children
My small son accompanied me on hajj. Is his hajj valid and does it fulfill the obligation?
Ibn 'Abbas (may God be pleased with them both) narrated that the Prophet (peace and blessings be upon him) met some riders at Al-Rawha`. He asked them: 'Who are you?' They said, 'We are Muslims. Who are you?' The Prophet replied, 'I am the Messenger of God.' At this, a woman lifted a child and asked, 'Is hajj valid for this [child]?' The Prophet replied, 'Yes, and you will be rewarded'” (recorded by Muslim).
Your son's hajj is valid provided you have completed its integrals and duties on his behalf. As he will earn the reward of a voluntary hajj, it does not fulfill the obligation because obligatory acts of worship are only mandatory upon a legally accountable person, which a child is not.
And God Almighty knows best.