Well, I have never seen anyone take extended time on the AMC 8 or AMC 10/12 but that just may be because we always took it at a university site and not at a middle or high school. If your DS needs extra time, it certainly can't hurt to ask but you might want to do it ASAP as special arrangements may require a separate room. In our experience, the minute the time is up and papers collected, loud conversations erupt.
I don't remember your DS' age, but it is also possible that he may not need extended time if there are a sufficient number of problems that are beyond his current ability. Try one of the old tests on the AOPS site to see how much timing is an issue. For example, I was surprised that last year for DS, time was not an issue on the AMC 10 even though sometimes he could have used more time on the AMC 8.
