(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?
You can also say "in the workplace": 25,000,000 googles!

That's a good explanation, but there are so many exceptions that you can't rely on it in all cases.

In fact, you can say "He is at his office", or, more likely, "He is at the office". "in" makes it more specific, but "at" is also possible.

Teachers: We supply a list of EFL job vacancies
Thanks CJ.
 MrPedantic's reply was promoted to an answer.
Thanks MrP.
Students: Are you brave enough to let our tutors analyse your pronunciation?