(1) He is at his work place.
(2) He is in his office.
In first sentence, the preposition 'at' is used because 'work place' is a generic place where certain work-related activities take place. On the other hand, in the second sentence, the preposition 'in' is used because office is generally surrounded by all four sides or in a three-dimensional space.
Is my understanding correct in the given context?