I thought Brutus was his son and his last words supposed to be “tu quoque fili”

Caesar's last words, according to some accounts, were "Kai su, teknon?" - "You as well, my son?"

It's generally accepted that "my son" is meant figuratively here, as a younger man whom he considered affectionately, but some argue that Brutus was literally Caesar's illegitimate son from an affair with his mother.

This is generally considered a fringe theory, as it would require Caesar to have been in his mid-teens when he seduced Brutus's mother, but is not completely impossible.