I want to nominate my advisor for an advising/mentoring award. I always use my advisor's first name when I talk or write to him, and as far as I known most students in my department and university do the same with their own advisor. But would it be appropriate to do so in a nomination letter? It feels weird and distant to refer to Professor Smith, but I also fear that John may be seen as too informal in this context.
I would begin the letter referring to him by his full name with title and after this I would then go on to use his first name only. It implies that you know him well, allows you to easily change the tone of the letter to be more friendly and it's the way you're used to addressing him. For example:
I am writing to nominate my supervisor, Professor John Smith, for the XYZ Mentoring Award. John has been my supervisor for the past three years and his mentoring during that time has been second-to-none (etc.)