I'm concerned about my friend, he keeps taking on new personalities?

[Dylan]

I'm kinda worried for my friend, he's 15, kind of a misfit, small group of friends.
Anyway, he's been doing this for about 5 months, often there's a link between stress and him doing this. But what he does, he makes fake accounts on Facebook, it's sometimes not obvious when he does this, he goes to extreme lengths to cover up any things on the accounts that might make it look fake.
He literally keeps these accounts updated, he adds the people he knows (that's sometimes what gives it away). But what's really weird, is that he gets fixated on the 'personality' of the fake accounts, he changes the way he acts slightly with each accounts, sometimes tiny changes, sometimes completely different.
I'm just kind of concerned, he shrugs off any people concerned, saying that there's nothing wrong. He claims he does the fake accounts for a laugh, to troll (well, fail at trolling) or "for the lolz".
it's just, i'm concerned, is there a medical condition that might have afflicted him, is this a way of venting something like depression, or large amounts of stress?
He only does this on Facebook, not in real life (although i don't talk to him ALL THE TIME, so i wouldn't be really aware of any personality changes in real life).
So yes, is there any medical conditions that can cause this behaviour?