Is my saying to my brother, "There are people with ignorant mentalities like you," considered an act of disbelief (kufr), and what should I do about it? And did my brother commit an act of disbelief by saying or insisting on repeating song lyrics after being informed of the fatwa? And are there any consequences for him after fully understanding the matter?

Describing someone who does not accept ijma' (scholarly consensus) and qiyas (analogical reasoning) as ignorant does not necessitate kufr (disbelief). Nor does your brother become a disbeliever by his statement as long as he intends to absolve Jesus, peace be upon him, of imperfections. Rather, it is an act of supporting the prophets to absolve them of the imperfections they are accused of. However, it is forbidden to go to extremes regarding them and to describe them with attributes of Lordship. It is advised not to delve into these matters before overcoming obsessive thoughts (waswas).