Ideally we would need 15 recommendation letters, from tennis schools (the more prestigious the schools is the better) that are willing to say he has extraordinary ability as a tennis coach.   Maybe one or two famous politicians or famous celebrities to write that he does private lessons for their son or daughter (that is only if he runs out of schools because schools are the most important.  We would want variation of letters, some from school outside of United States and some schools in United States.  We would help prepare the letters, but we would obviously need information from the school.
For example, we would want information such as the school teaches 500 students year, they are prodigious tennis school, some of their students have gone on to play professional tennis, and provide the school’s website. (We understand schools will be able to say that but we would need as much information as possible for us to school to write a detailed draft of a recommendation letter)
Obviously, we want to provide all the documentation of his success as a tennis player.  All awards, pictures of trophies, articles about him in English, and any other language which would need to be translated.  We would also need some basic information about the school that is sponsoring this person.
With the aforementioned you would have a strong chance of obtaining an O-1 visa.
*** New York Immigration Lawyer Andrew P. Johnson was a former prosecutor and has exclusively practiced immigration law of over 15 years.  The Law Offices of Andrew P. Johnson is located at 65 Broadway, 21st Floor, New York, NY 10006, phone 212 693 3355, email