A strong swimmer shouldn't fail but 2 minutes treading water then 2 lengths wearing a boiler suit isn't easy. Dont worry though, if he doesn't drown he will be given loads of opportunity to take the test again and again and again until he passes.
thought that was part of the test for pool lifeguard..... not the mil swim test. presume your meds haven't kicked in yet?
As for the OP, establish why your son failed, as another poster said, he may well have touched the side at some point or failed on some other technicality. I would advise him to listen to Instructions very carefully during training, getting it right the first time, saves a lot of energy!