It could have been anything. Dogs don't snap for no reason, so something set him off that you missed. It could have been a lot of small things added together to put him over his threshold, ie he was hot, tired, had a headache etc the boy may have smelled funny to him or reminded him of someone he didn't like, you may never know. In future watch out for any subtle signs of stress, lip licking, looking away, tension in his tail or body, 'whale' eyes. Not all dogs make their discomfort in a situation obvious by growling etc. I used to know a very reactive collie and the only sign that he was ready to snap was a tiny bit of stiffness in his lip, like a micro lip-curl. Once you knew what to look for it was easier to prevent him biting.