If you are staying within the Apple ecosystem (everybody he might text has an apple device), then just use iMessage. You can add whomever you want to the Contacts app. This doesn't prevent him from texting someone else if he knows their email address and they are not in the Contact book though.
Otherwise there are lots if texting apps like TextPlus, WhatsApp, Kik and others but I don't know of any that completely lock down to a known contact list. The lists are more for shortcuts so you don't have to remember email addresses or phone numbers rather than locking you down to only entries in the contact list.