Hi, and welcome
It is possible it is not lasting the entire 12 hours, everyone metabolizes differently. It is also very possible he has asjusted to it and it is time to raise the dosage. Meds are trial and error, and raising the dose and changing meds
A child may HAVE ADHD, but it is not what they ARE. Never tell a child they ARE ADHD.
My 10 year old is on Vyvanse but on 40 mg a day. We saw some regression each time before the dosage was upped some. We have been at 40 mg now for about three months and it seems to be working pretty well for the full 13 or more hours. I'd talk to the prescribing doctor see if he needs a higher dosage.