"Today I am a boy, but tomorrow I will be a man."
Translation:Hoje sou um menino, mas amanhã serei um homem.
Serei is the conjugation for "eu" (1st singular person) and será for the 3rd singular person (ele, ela, você, isso, aquilo, etc)
Why is ‘ser’ used here instead of ‘estar’ even though the sentence makes it clear that this is a temporary kind of ‘to be’?
Maybe it will help you to memorise it if you think of these states as being similar to jobs. Jobs aren't necessarily permanent but people use "ser" to describe their occupations. Even a student (definitely not a long term thing) would say "Eu sou estudante".