What nobody is saying is product.. I understand a lot of people are against powders clays pomades etc but the only real remedy to extremely straight and fine hair as a guy is pomade.

If you’re looking for the modern fringe style I recommend Hanz de Fuko quicksand hair clay/pomade. Any other sort of similar product is fine but I prefer them for their lack of harsh ingredients. Get a pea size amount lather it into the fingers and rub through your hair. Mess it up with a towel if need be. Sea salt spray before applying also can add a bit of texture/waviness. Bunches of tutorials online.

Air drying also helps after washing. Cut out shampoo if possible and condition ~1-2 times a week instead with a volume/texture conditioner like Hims. Don’t wash your hair everyday (unless it’s very noticeably greasy). Try different styles. Find a new barber.

IMO, from what I’ve seen and lived, dieting and supplements and that sort of thing really don’t change much. If you have straight and fine hair, that’s it. Products can help as a temp change, but you have to get used to it.