What assessments to give?

I have a 5 year old kindergarten student who is currently being reevaluated. He is severely unintelligible-either phoneme collapse/severe phono or possible CAS (trying to figure that out). However, his receptive language skills are all within the average range. He also has an AAC device that he is super independent with. I completed the communication matrix, but it didn’t tell me much because he has almost all of the skills already. He doesn’t use grammatical structures yet (plurals, -ing) so I don’t know how much I’d get from something like the OWLS. What else would you use to test expressive language for an AAC user? Also, how would you informally assess CAS vs. phono other than consistency because we don’t have any Apraxia assessments currently. TIA!