He also notes in the mage path/future redcliffe that he CAN die, in fact he doesn't really care if he does. So I figure if there was a choice between joining his loved ones or not he'd choose to go with them.
*Now I'm sad* Dangit Solas why can't you value your own life? It would be much easier to woo you if you did. Stupid noble selfless elf. Even his good qualities are adding to why he's making bad decisions.
Oh yes, Solas CAN die, but he doesn't age. So for Solas to join Lavellan in the afterlife (so to speak) after her life ends he would have to kill himself. And he would still need to watch her age first.
It's stuff like this that makes me think, in a Solavellan romance our Lavellans may be in the best position to get him to stop his plans, in a way, but also, in a way, being in love with Lavellan might actually be making him even more determined to "fix the elves".
But like I said, I wasn't trying to sway anyone here. If for anyone's Lavellan they see it as "better to have loved and lost then to have never loved" that's good! My Lavellan isn't so sure.
For some reason, I'm reminded of this quote from LOTR:
Ah yes, but she ended with a mortal lifespan after choosing to stay didn't she? I'm not sure Solas gets that option.
Yeah, this notion has certainly passed through my mind as well. If Solas is immortal and his beloved and descents are not then he will need to "endure" a good deal more loss. Outliving your spouse, children, grandchildren...is a very depressing thought from our perspective, but try to think of it from another point of view. Would a parents like to be able to care for their child their whole life? To see ALL their milestones, to hold their hand in times of grief and pain? I've occasionally thought how odd it will be to lay on my own death bed without my parents there to help me through it. It might be a weird thought, but it happens in my crazy brain. I've wondered if they feel some guilt for possibly leaving me alone in the world some day. Solas would never have to leave his descendants, and along with the pain would come a great many joys.
In exchange for the grief he would have to endure he would have the rare privilege to be the caretaker, the voice of wisdom, the constant comfort for generations of his own family.
I'd still prefer my Lavellan to share forever with Solas, but if that's not possible then I could still see this alternative as a happy ending.
Oh trust me, I see it from that point of view too. As I said, I wasn't trying to change anyone's mind or sway anyone on their thoughts for how their Lavellan should proceed wish a Solavellan ending. Just sharing my Lavellan's opinion and her state of mind on the matter. 
If you see that as a happy ending or not is a matter of opinion, to my Lavellan it's not, to someone else's it might be. And I have no idea if it is to Solas because I don't write him. But my guess is that it's not, considering he's already horrified at outliving all his friends, but that's just MY interpretation.