Sean Ben Noach:
Really the only answer to your question is that the chukas hagoyim issue is clearly not related to wearing shorts. Only one poster brought it up, and he is mistaken. Chukas hagoyim stems from things at least tangentially related to idol/foreign religion worship or promiscuity. The correct issue regarding this subject (boys in shorts) is tznius, not chukas hagoyim. And that is the same question regarding wigs.
Good luck!