We have Nick (7) on 36 mg of Concerta.
Concerta is supposed to be a 12hour med, but some kids do metabolize it faster than others. I , honestly, would go to a higher dose first.
A child may HAVE ADHD, but it is not what they ARE. Never tell a child they ARE ADHD.
You can actually go next to 45 mg before leaping to 54 mg. Our six-year-old takes one 27 mg and one 18 mg in the morning. He had a horrendous start to the first grade, with lots of bad reports in class (fidgety, couldn't sit still, unfocused) and two trips to the principal's office. Bumping him up to 45 mg made a HUGE difference and now we're seeing Concerta last longer than it ever has ... nearly to bedtime (used to konk out around 3:30/4:00 pm). Within a month of the increased dosage, he was his classroom's student of the week, which is based on good behavior (and he was only the second boy to get this recognition at that point).
You might consider the booster, but if his performance in the morning isn't that great, a small bump up in the dosage of Concerta might help. I would definitely give your son's doctor a call for input on the situation.
I'd be considering a dose increase, too, but to 45 (my kid did it just the way the previous poster did). If, um, your kid is realllly sensitive to the meds...well, mine can feel the "dips" in between the maximum dosages. It's generally our first clear sign that a med raise is going to be needed.
I've always tried to avoid having to have him get a booster in school. Way more trouble, I'd bet mine wouldn't remember to go get it, that kind of thing.