We tell our MathCubed students that they should have a solid understanding of the math concepts at least five days before the exam so they can focus on reinforcing the math material in the days leading up to the exam. In other words, they aren't trying to learn anything new at the last minute; they are reinforcing what they already know.
If your son starts his math prep more than a week in advance of the exam, that will give him enough time to work his way through the three essential exam prep steps so he can identify and strengthen the math concepts he finds difficult and, if necessary, ask his teacher for extra help. If he gives himself enough time and does a review following the three essential exam prep steps, he will find this a much more effective way to study than trying to cover all the math from the whole course in the last day or two before the exam.
CRAMMING
The key problem with any student studying at the last minute is that it leads to cramming which is usually an attempt to memorize math questions without fully understanding the math concepts they are based on. So, even though the student is investing time doing exam prep work, they aren't able to retain very much and they usually realize this very quickly. The result is the student begins to feel discouraged because none of what they are studying is sticking, and when that happens it's difficult to stay motivated.
To summarize, it might not seem exciting to start studying weeks before the exam, and it definitely takes some discipline, but if your son is motivated to do well (and follows the three essential exam prep steps), he will find the investment of study time really pays off.