Can one fight medical rejection due to depression noted 14-15 years ago?
There are Aviation Medical Examiners (AMEs) who specialize in special issuance medicals. It is probably best to use one of them instead of your regular AME. This type of special issuance may require a battery of psychological tests and it may get expensive. But the good news is that once he has his special issuance, he can fly under Basic Med, so he only has to jump through the hoops once.

I know of two who are regular participants on pilot message boards and they are well-respected.

Bruce Chien, MD http://www.aeromedicaldoc.com 

L B Fowler Jr MD lbfjrmd@gmail.com.

Edit: I just ran across a fairly long post on Pilots of American that addresses a similar issue with a good (but time-consuming and expensive) outcome. 


  I wanted to share my story of a difficult SI in the hopes that it will
  give others some insight and perhaps courage to not take 'no' for an
  answer.
  
  I had been diagnosed with mild depression during my senior year of
  high school 25 years ago. The psychiatrist I had been sent to put me
  on Zoloft (Sertraline), which I guess helped. I stayed on it via my
  family doctor for the years afterwards, not really giving it much
  thought, and never realized the difficulty it would cause in working
  towards a PPL someday. …