Siddhartha is a Brahmin’s son? What does that mean? Why is Siddhartha unhappy?

Yes Siddhartha is a Brahmin's son. A Brahmin is the highest level in the Hindu Caste system. Siddhartha realized that none of the people or material things around him were capable of giving sustainable peace and happiness. He wanted to find true peace that was not based on external validation of things and people.