Show him the marathon runners that water fast and use the keto diet to increase endurance. I forgot his name but I think one of them was on the JRE years back.
Take care to show it requires adapation. Beginners wont get a boost: https://pubmed.ncbi.nlm.nih.gov/8248683/
Then show him the bodybuilders who use fasting to promote insulin sensitivity
Stop talking about fasting as the main point and talk about the mechanisms fasting already affect. Without making that connection he cannot have a thought because there is nothing there to think about.
Theres more as well but it all boils down to fasting leveraging mechanisms. You use those mechanisms to specific purposes just like you use the mechanics in mind when planning your diet and exercise plan to specific purposes.
educate him or rather have his stubborn self watch the nobel prize in 2015 from autophagy. no other modern medicine including hyperbaric oxygen chambers or occlusion resistant training or vasoconstriction therapy if dude is really a sport science person should kno those things, none can match the healing process of fasting especially dry fasting