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 met some riders at al-Rawha` and asked, 'Who are these people?' 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" [Muslim].

The ruling

Your son's hajj is valid provided all the obligatory actions and requirements are complete. He will earn the reward of a voluntary hajj but it does not fulfill the obligation. This is because obligatory acts of worship are only mandatory upon a legally accountable person, which a child is not.