What do you say when you are going to make sure nobody else takes it?
Like you are going to occupy the seat for him.
Sorry if it was a bit unclear...
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.
Sign up to join this community"I'm holding this for my friend, he'll be back soon." Or anything to that effect that is succinct and to the point.
If no-one is actually trying to take the seat, you do not have to say anything. If someone tries to take the seat, or if they ask if the seat is available, the usual thing to say is:
I am sorry but this seat is already taken.
I'm sorry but my friend is (already) sitting here.
I'm sorry, but this seat is taken.
As a related bit of culture, in the US we often put an article of clothing (like a jacket) or a purse/backpack on the seat to indicate the seat "is taken".