Translation:The boss needs a desk in his office.
It is most likely talking about the boss's office. If the context is there, it could also be someone else's office, including "your office" for a person you address with usted. It's not formal because you're talking about the boss. Formality has only to do with the people you're talking to.
(But why would need the boss a desk in someone else's office?)
Is there any reason why "The boss needs a desk in your office" is incorrect? Since we have no context for the sentence both your and his should be correct, unless "the boss needs a desk in your office" could be understood as a command and necesita would be conjugated differently.
This is very confusing when you grow up learning Portuguese and eventually start studying Spanish, because this sentence would be translated as "the boss needs an office in his car shop" (o chefe precisa/necessita de um escritório em sua oficina). Then I have to remember these are two false cognates. lol