Bureau menas both desk and office. So 'il est à son bureau' means he is at his desk or he is in his office?

Hi Emma,

 

Using the preposition "à" in "il est à son bureau" means that he is at his desk. However, to say "he is in his office", you would use the preposition "dans"= "il est dans son bureau". To say "he is at the office", you would use the article "au"= "il est au bureau."

 

Hope this helps!