What is the ruling on my telling my younger brother Abdurrahman that calling him "Humani" is forbidden, and was I correct in doing so, or would it have been better to say it is impermissible or undesirable?

The Salaf (pious predecessors) disliked the pronouncement of "haram (definition)" (forbidden) and "halal (definition)" (permissible) except for what was definitively proven by the Qur'an and Sunnah (definition), based on Allah's saying: "And do not say about what your tongues falsely describe – 'This is lawful and this is unlawful' – to invent a lie against Allah. Indeed, those who invent a lie against Allah will not succeed," and His saying: "Say, 'My Lord has only forbidden immoralities – what is apparent of them and what is concealed – and sin and oppression without right, and that you associate with Allah that for which He has not sent down authority, and that you say about Allah what you do not know.'" It is more appropriate to say: "It is not befitting."