"He is in the building's entrance."
Translation:Il est dans l'entrée du bâtiment.
"il" can be "he" or "it" : a man , or an inanimate thing or an animal is in the building's entrance.
But if the English is "he", you have no choice: a man is there.
"he is..." turns to "c'est..." ONLY if followed by a modified noun, ie an article (or possessive or demonstrative) + a noun describing what the person is:
Please take a look at this: http://www.frenchtoday.com/blog/cest-versus-il-elle-est
I followed that link, and it reinforced things I feel I already knew; but it didn't help me understand the times that are WRONG to use c'est.
I found this link - http://www.french-linguistics.co.uk/grammar/c_est_il_est.shtml - of more use. However, it seems to suggest that while c'est would be technically wrong on this sentence (because of the clause that follows), it is common in everyday use.