I wasn't aware that KORI (which is a mallet instrument manufacturer, by the way - the American branch of the Japanese company, "Korogi") makes bells; I've only seen their wood-bar instruments (marimbas, xylophones, tabletop "practice" xylophone):
http://www.custommusiccorp.com/kori/koricat.htmlMost of the bell kits I've seen (in stores, advertised on the 'net, etc.) have been from the brands, Musser or CB-700.
Cutting to the chase, if your son gets really serious about mallet playing, he's going to eventually need a xylophone, marimba, or vibraphone (most likely marimba if he goes for classical studies, vibraphone if he goes for jazz studies). Beyond their use in school band programs, orchestra bells are a fairly limited instrument for use in mallet studies; there just isn't enough literature (textbooks and compositions) for students to work on. (There are bell solos, folks, including some hip things by George Hamilton Greene's brother, Joe, but they're a rare breed indeed.)
Also, the bells one finds in these student "bell kits" are not professional quality - even if you buy yours, you'll end up selling it at some point anyway if your son gets serious about performing on or studying mallets. (That may be a viable option - buy it now, then sell it to a beginner at the school when your son is ready to move on to another instrument.)
My humble advice? Since your son is
required to have a set of bells, if you can buy for the same cost as what the rental fee will total up to, then buy, and sell it later. If not, then rent, and put the saved money towards eventually buying a student-level marimba, when/if your son gets serious about mallet playing.
