Ski rental office? If your kid is big enough, maybe you and he could just rent the clothes for a day or two at the ski rental shop on the mountain, if there is one.
I had the same problem, ski clothes to wear once a year for my kid, so I picked up a lot of great stuff at Marshalls (the one in Irvine). Don't know if they still have anything, but worth checking out, I got ski jacket for 20 bucks & special ski pants for 16.
Apparently Chick's sporting goods is turning into Dick's Sporting Goods. Does anyone else find this funny? In college I was always taught by my girlfriends that it should always be Chicks before Dicks. I guess it's true in more than one way. Disclaimer: I'm easily amused.