It is not obligatory for men to cover their heads during Salah or during any other act of worship. It is permissible to offer Salah behind a person who does not cover his head, because the head is not included in the `Awrah (parts of the body that must be covered in public) in the case of men. (Part No. 6; Page No. 172) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.