Hmm, I suppose you're right. I was going off by how I felt during the day.
The outside of the concerta pill apparently contains 22% of the methylphenidate; this is released immediately. Over the next ten hours, the other 78% of the pill is released. I don't know if you or him have ever split up one of these pills before, but they're pretty crazy: there's the outer layer, a waxy layer beneath that, and then the interior of the pill is made up of three colors: green, pink, and white. I'd guess that the green layer releases the slowest, as when it is combined with water it forms a very sticky substance. But anyways.
I would often only take concerta during the weekdays and never on the weekends, because I felt like I should only use it when I absolutely needed to. I felt absolutely fine on the weekends, except for maybe a little irritability. So far as I know, withdrawal symptoms don't occur with most patients, but of course, everyone is different. You should have him try taking it during this week and then not on the weekend and see how he feels, just as a test. He should feel fine and have no symptoms at all, but if he does, it's suggested to taper off the dose. Now: he is taking a fairly small dosage, so I doubt there will be much of an issue.
I've read up a little more on the combination of stimulants and Concerta: other than a possible increase in heart rate, there is an issue of cross tolerance, meaning that if you take Concerta on the day where you plan to take stimulants later in the day, it may lessen the effects of them. I have done MDMA on an evening where I had taken the Concerta in the morning and didn't really notice any difference.