When I think about it, I could have just said he was part of a displaced ashlander tribe like the cantemeric velothi; after all, the Zainab show that one doesn't have to be a miserable git to be a dark elf. But I'm too attached to this decadent Dres idea now.
So firstly, what's his name? I think it should be a house dunmeri name, but bosmericised. I can't think of any Dres names in vanilla Morrowind, but that's alright because they've been in Valenwood so long, they probably don't have any living relatives left elsewhere.
And I've thought of a 'born to uncertain parents' idea. He was a bastard born to an otherwise respectable (wood elven standards) maiden with the characteristic red eyes 'the clan' has in infancy, so she knew it was one of theirs and they were happy to take him back (the're tightly knit like that). They could probably use a simple bloodletting ritual in their sleep to find the father. But that would screw up 'born to uncertain parents', so I need a reason for why they don't want to. Maybe they have their own nerevarine prophecy (with goals which concern their ancestors rather than Morrowind [which might be a load of tosh Azura told them]).
And I think that's it for now. I've got the class sorted, and he'll probably join the temple, the thieves' guild, the skaal and perhaps the morag tong, without coming to lead any of them. Any thoughts? Speculative bosmeri theology?