Because you used ELE, so "que" is referred to that. If it were not there, QUEM would be perfectly suitable
Did you mean "quem é o menino que cozinha" or "é o menino quem cozinha"? Thanks. 10/04/2017
Actually, I was replying to a person, but their sentence is no longer here, but both suggestions you provided are correct =)
Thank you. :)
But I thought "quem" is used only for questions and placed at the beginning of the interrogative sentences? however "é o menino quem cozinha" is an affirmative sentence...
"Quem" can be used alone as an interrogative pronoun "Quem é você?" but is preceded by a preposition when used as a relative pronoun. The link has examples: