Firstly point out the meaning of FAD - an intense and widely shared enthusiasm for something, especially one that is
short-lived; a
craze.
Secondly remind him of William Bantings booklet called
Letter on Corpulence, Addressed to the Public and written in 1863. Maybe refer him to
https://en.wikipedia.org/wiki/William_Banting, just to fill in some gaps like why he followed the dietary regime he did.
Even as a teenager, now over 50 years ago for me, I remember mum dieting and the first thing she did was cut out bread and potatoes. So this low carb lark isn't so much of a fad really and certainly not short lived. Maybe he can explain why in his ignorance he can say that something that has been a round for over 150 years is in his mind a fad.
BTW, I recommend reading about Banting, and the physician (Dr William Harvey) who recommended a low carb diet after attending lectures in Paris about Diabetes. The Frenchman (Claud Bernard) who gave the lectures is interesting as well.
I think he needs to have some empirical evidence to backup his BS. He's entitled to his opinions, but he can't just grab ideas out of thin air. He needs the evidence, maybe he can suggest some further reading?