Does this mean "he is always the last one to arrive at the office" OR "he is always the last one at something (f.e. productivity) among all the people at the office" OR both?
I would say it could mean he is always last at something, or the last one to leave (if he was the last arriving you'd have to say Han er alltid den siste til å komme på kontoret. )
If you want to tell someone You're the last (over the finish line)/You came last = Du er sist/Du kom sist. You can use both Være and Komme to describe this, they're not always interchangeable though.
Hvorfor er det 'siste' her? Er det fordi ordet 'person' eller 'man' ble savnet? 'Han er alltid den siste [mannen] på kontoret'