I do know when one is tired, no matter how much training you have, some natural tendencies will slip back. I am an adult, and yet I know for instance when I am really tired or sick, I forget some basics of the speech therapy and I will start to slur some sounds or drop some sounds again - despite the 10+ years of speech therapy I had as a child. My HS speech therapist (oh so many years ago now) mentioned that he noticed my tendencies to forget what I clearly know under certain specific conditions, and would have me check in every week or two to "remind" me and touch base on what I am up to - not for any formal therapy since I did not have speech issues, when I was not in those scenarios he noticed, but to sort of touch base in case something did change without notice.
Maybe, your son could use some resource that he could once in a while just check in for refresher on what he learned and just touch base?
