You're doing a great job already.    If he doesnt express it it means the world to him to have someone like you in his corner. Unfortunately, there is no cure.  At best some exercises can temporarily help for the day (I.e. read with pencil in the mouth, tongue swirls, etc) with repeated exposure to build confidence through experience while accepting the issue and not trying to find a magic cure or waiting for that magic moment to happen.  Good luck to him and much respect to you.  You're a terrific sibling.