I think it's believable. If an arrest has been made based on the victim's story and surveillance footage then something must have happened.
However... There is still that nagging element of doubt. Why would anyone expect a teenager (well, under 18) to be able to buy a gun? You have to be at least 18 for a long gun and 21 for a handgun, so the criminals would likely have been out of luck.
I would also like to know why he didn't put up more of a fuss once they had him in a Wal-Mart. I suppose if one of them had a handgun, they could have shot him anyway, but it would have been a opportunity to get away since it was a public place, and a large one at that.