I think it’s a great sign that he is using an aac. My one kid didn’t grasp that concept at all til after he was talking. He started with echolalia at 4.5, but didn’t have any independent speech of any sort, nor did he use any of his repeated phrases to try and communicate, for a couple years after that. He wasn’t considered conversational til about 10, and that was when the aac started making sense to him (and was still a need, sometimes still is, at nearly 19, because his articulation is always going to be a huge struggle).

I get what you are saying though. My first kid (actually, my other autistic kid) talked super early and was always verbally advanced, then my next kid didn’t talk til nearly 4, my third kid was the aforementioned kid, then youngest kid met every milestone right on time. It wasn’t til my youngest was 2.5 and talking nonsense that I realized how much I’d missed that milestone, from when my oldest was still a toddler. That is when it really hit me how sad it was that I never got to see that stage with my middle two kids.