Isn't the correct English equivalent "THAT is not a man, but a boy."?
Ten instead of to would be an unusual sentence. (Possible, but in a specific context - "That one is...").
This is the extremely common way how we use the so called copula (copula = linking verb, connection verb) sentences with a noun predicate. In Czech these sentences always use TO for all genders and numbers (TO JSEM, TO JSI, TO JSME, TO JE, TO JSOU, JSOU TO, JSEM TO,...). Why use it then? Because that is how it is done. It is the normal way. You cannot just put something else there.