For these kids, I continue to work on verbal speech, but pair AAC with his verbal speech in the meantime to allow for communication success and less frustration. The research has consistently shown that pairing both increases verbal communication. It will be obvious, as you progress in therapy, if this child will be a verbal communicator. Even if he ends up using mainly AAC, at least you can say you tried.
What are you using for a therapy model? Cycles or apraxia based program?