Buying second hand is always a gamble... it requires knowing the seller well enough to trust them to stand up for you when any trouble starts. Here is something that the seller needs to do to make it a legitimate sale...
Reselling Adobe Software
As far as whether or not your son will be able to use it, that is not clearly enough stated to be able to determine whether it is a question of...
1) is your son capable of using the software - no offense intended
2) will the software be of any use for your son as it relates to his schoolwork or interests
3) is your son's system cabapble of running the software... as a minimum, see if the trial version works before buying (Download Adobe CS5 Free Trials – All Direct Links without Akamai | ProDesignTools) - if so, just use the serial number from the purchase to activate the trial to full use
4) there's always one more than you can think of...
What you might consider instead of a second hand purchase is to help your son purchase the student version of the CS6 software instead. The discount you will get (I think it's usually about 60% off) might make it worthy of consideration, especially since it will be a more up to date release of the software.
For details regarding the student/teacher provisions you can refer to the following information page:
Another good resource for getting information is the student and teacher edition information forum:
Here are some links regarding student discounts from Adobe:
You're welcome Mike