Why not quel beau homme?
Because of the vowel conflict, "beau" changes to "bel" before a word starting with a vowel sound?.
The same applies to "nouveau/nouvel" and to "vieux/vieil".
Why don't you use an article here, like: "Quel un bel homme !" ?
Actually "quel" is a kind of contraction for "que" + "le", so you don't need another article.
Ohh I see, thanks :)