As far as questioning whether that's really his sexuality, how he knows, etc... I wouldn't question it. Even if by chance he does find that another sexuality fits him best in the future, it's for him to figure out. Multisexual (bi, pan, etc.) people already tend to face doubt and questioning from others, so I think it'd be best just to believe he knows himself best. As far as what it means, people tend to define it different ways. I was actually around his age when I realized it was the label that worked best/felt most right to me and didn't have much experience either, and it still hasn't changed (at 23).
I think you've got it exactly right when you say that you don't need to fully understand it to support him.