On sports autographs, JSA and PSA/DNA are the most widely accepted 3rd party authentication companies.
There are other very good authenticators out there including Richard Simon, Mike Guiterrez, and Kevin Keating to name a few who mostly specialize on sports autographs although they also authenticate other autograph categories as well.
There are many experts on this forum on sports, entertainment, music, etc. as well.
Is this Scott Stimell of Cardboard Memories? I think he used to work with JSA consulting on sigs. I know he does private signings, too, and is one of the "good guys" when it comes to the sports hobby.