Il est un homme is right!
No it isn't. "Il est un homme" is a big grammatical error in French.
why is it wrong? i don't understand what it means for a noun to be "modified"
takoknight is right.
You have to use "c'est" when there is an article (une, un, le, la, or l') or a possessive (mon/ton/son, ma/ta/sa, notre/votre/leur) before the noun.
I vaguely remember (from school, ages ago) that if the predicate of the sentence has a reflexive noun, then you use c'est...