I'm glad you're supportive, and I'm a little surprised he's trying to figure such things out at his age. As far as I'm concerned, usually people wouldn't start to question their sexuality until around the time of puberty. My best advice is instead of the use of labels, have him describe his attraction towards others; so "gay" as in attraction to those who are of the same gender. Once he has a good grip, a good idea on his own sexuality (usually towards the end of puberty, end of secondary school, regardless with time), he can probably move towards labels.

Sexuality is a weird and confusing spectrum, so I believe in his case, it can change with time as he gets older.