A deficiency test is what I would have recommended.
My husband started losing hair rapidly at 20. He had bald patches above his forehead, and around his temples. He was very distressed by this, because he would have been ok with hairloss at a later age, but he wasn't ready for it. He actually spent a lot of money on doctor-prescribed hair loss formulas in order to combat this. After a year he decided to let nature take its course, because the treatments were so expensive (around $50/month). They did help though.
After he decided to embrace it, his hair loss actually capped out and stopped almost completely. The fluff in front fell out, and he was left with a hairline about 2 inches further back. He's now 30, and he has no bald patches, and his hairline is further back but still natural; it looks great. Just like this...I honestly think it's a really good look.
So...don't worry about it too much. Rapid hairloss often takes place 18-23 or so, then slows dramatically. It often looks better after the wispy parts fall out. There is a good chance that it will settle down and continue to thin at a more comfortable pace. If not, you're not alone. I can think of many handsome friends who are bald or balding; often they go with a very short cut, which gives the appearance of thickness. Don't hold onto that long stringy hair if you have it, whatever you do. My husband did at first, and oh my...not a good look.
Lastly...he doesn't read these forums so I have nothing to hide. I actually think he's hotter now at 30, hairline and all.