What is the ruling on issuing a certificate proving a man's Mahram status to his mother, and what are the motivations and benefits of doing so?

Issuing a certificate of proof of mahramiyya (being an unmarriageable kin) is permissible if it accurately reflects the reality, and it takes the ruling of the matter it pertains to. The motivation for it is to obtain a purpose that requires proof of mahramiyya. Among its benefits is avoiding accountability or punishment resulting from not proving it.