I'm pretty sure that any conversation between Solas and Fenris would end up in Fenris trying to spirit-punch Solas through the chest. Though it could be enlightening for Solas provided he survives the experience
A face to put to all the modern elves' troubles, so to speak.
I don't know. Fenris doesn't really relate to his being an elf, as evident in his banters with Merrill. So long as you don't call him flat ear, and I doubt Solas would, I think they'll be pretty chill. And who knows, maybe Solas will even offer to remove Fenris's markings and then I no longer have to think about Fenris someday dying of lyrium poisoning
Edit: Ok I can't help but think about this some more.
I think Fenris is going to be on edge at first, as he always is with a mage. But Solas is polite and knows when to keep his distance, though he'll probably be both disgusted and fascinated by the lyrium markings, since they're probably Tevinter twisting the vallaslin. So little by little, he'll try to get close to Fenris. And Fenris is actually pretty civil, except when he hates you. So unless Solas shows his pity, and he knows when not to as evident in his relationship with Sera, and so long as Solas doesn't try making Fenris more elfy, they'll be fine. Heck, they can even be friends.