well the big advantage of carbon fiber bows is that they are pretty much indestructible.
http://cgi.ebay.com/*GERMAN-STYLE-H%...QQcmdZViewItem
i don't know the quality, but it's pretty inexpensive. really, the best way to know what you want is to let your son try them out and decide. having a good responsive bow is very important to developing solid technique (i think more important than having a good bass). at the same time, i don't know where you would find a truly high quality half size bow, i don't even know if they make them honestly, as any player using one will eventually out-grow it. keep this in mind though in terms of cost - this bow listed above is selling for about what i typically pay for a rehair. that tells you the hair is probably not very good, and maybe a rehair would be worthwhile to begin with.