There are a number of possibilities. Ordinarily charcoal lighter fluid is safe if used as directed. However if he splashed some on his clothes and then lit what was in the grill, he could have ignited his clothing. Had he put lighter fluid on a hot grill, it would vaporize rapidly producing a cloud of flammable vapors. Or he could have been using something much more dangerous like gasoline. Can't rule out the possibility that he was experimenting. A friend lost a son when he lit some black powder in a backyard BBQ. The powder flashed and caused extensive burns that led to his death 6 hours later. (His son was in his mid 20s at the time - not an impulsive teenager.)
It seems clear that he had some on his clothing and provided a source of ignition. These are terrible mistakes to make and can extract a heavy price.
In a similar vein, I cringe when I see a Weber on a wood deck or inside a screen porch. That's just too close to burning the house down for my comfort. I had a cook going when a terrible storm hit (which left us without power for 3 days) and hesitated to pull my cooker into the door of my garage. I did, finished the cook without leaving it and pulled back out when done.
I hope that news of this causes others to think twice and avoid the same mistake.
Disclaimer: I worked for a several years as a combustion engineer at a major steel mill and also ran as a paid-on-call firefighter in two communities for about ten years.