If you buy the tickets at SoG, they are already activated. You can actually buy tickets via SoG before your trip, if your arrival time is a concern. Just go to their website and you can purchase the tickets that way. SoG will verify the military member's ID as part of the purchase process. This may, however, ruin the element of surprise you're looking for.
If you have your son buy the tix at his base, the tix will not be activated and he would have to be with you at WDW to activate them because they'll be issued under his name and he would need to actually use one of the tix.