What to write in an award nomination letter about your supervisor?
I am guessing you are writing this because you want to nominate him and you have had interactions with no one else that is writing for him or nominating him.

Typically such letters for discipline-wide awards are written by senior members of the academic community, unless explicitly stated in the call for applications that very junior researchers are supposed to do the nominations. Very likely what will happen is someone will see that you are his student and consequently write off your application for your advisor as a conflict of interest. If you are nominating your advisor for an award based on their research level and you are a graduate student, chances are it is not going to hold the weight that other letters will have and they will pass over your nomination.

This being said, if your letter is in a bigger file of people writing in support of your advisor, then it will carry some weight. If you are applying with a group of people that are nominating your advisor, it has a shot. If you are doing this, I would focus on the other contributions aspect of the call.