"Is he not human?"
Translation:Он не человек?
Surprisingly enough, the rules of не/нет distinction in Russian are not that hard for the English speakers since не almost always copies not's behaviour and нет does the same with no. You can remember it as a rule of Reversed T's (yep, just made that up) since word-final т/t appears when its counterpart lacks it.
In my mind, "Не он человек" doesn't make sense, because that would translate to "not him human," and I don't know of any native English speaker who speaks like that.
The most literal translation of "человек" is "human". However this word is also very common in everyday speech in Russian, while in English "human" has more of a scientific feel to it, so it's better to use "person" to avoid sounding like an alien. Also "человек" is not gender specific, so it doesn't really mean "man", but it can be translated as "man" in certain contexts when "man" in English is used as a synonym to "person", i.e. when it doesn't focus on the gender specifically and is used as a more generic term (like in "to go where no man has gone before"). So basically you just need to use whatever is more natural to use in English.
Because a person is not an "it." A person is not a thing, so it is better to say "she is a person," or "he is a person."