If your son is happy with the Ric strings then you can order direct from Rickenbacker:

http://boutique.rickenbacker.com/Strings_c_14.html

Otherwise, there's no law that says you have to use the exact same gauge or type strings. Find something close. You may have to adjust intonation at the bridge (adjust the saddles), but that's a task that inevitably will need to be learned anyway.