Sons of men is used in Ecc 3:21 in reference to Hebrews (sons of men is used in the New Testament as reference to the redeemed). Ecc 3:21 can not be reference to an animal “spirit” in comparison to a human spirit (Romans 2:29). The “spirit” in Ecc 3:21 has to be a reference to being physically alive, breathing, and con not be the same reference as the Holy Spirit indwelling in mans spirit (redemption) of the New Testament. Only a human being has a tripartate nature (Adam was created in the image of God and spiritually alive to Jesus before the fall). All human beings since Adam have a  body, soul and a spirit which is dead to God at birth (all born dead in Adam), and have an opportunity to accept eternal life in the resurrected Christ that rose from the dead. An animal does not make a decision to accept eternal spiritual life from Jesus, so can not have a spirit in the manner of man.