Thing is... EVERYONE is spirit made flesh, in one way, or another.
It's made rather clear through all the hints we get. Cole can be pushed into becoming "fully human" - Solas says this himself. And he gushes about Lavellan having "rare and marvellous spirit" or says (to approved Lavellans) "if the Dalish could raise someone with a spirit like yours... have I misjudged them?".
Then there's a poem Inquisitor reads to Cassandra in her romance scene. The book the poem is in is actually on Chantry's list of forbidden books, which makes it even more interesting.
It ends like this:
"The body opens, my spirit there. Not merely housed in flesh, but brought to life"
So, there you have it.
Also - one of the codexes in Shattered Library mentions that "those who have never manifested outside of the Fade" have an easier time finding the roots of it. I think Solas was close to that - as every ancient elf, he was more than a spirit, but too complex to just embody one emotion. With that said, I don't know if he was indeed only just a spirit - merely someone with something of a vague physical form, more interested with the Fade itself.
He was an explorer - he himself told that to Inquisitor. He said that he grew in a small village - that Leliana's agents have found, so it WAS a physical place - and there was not much to do there (what is curious that he says "especially for a young man gifted with magic". Weren't all elves magical then? Or were there elves with especially strong magical gift? I mean, the game goes out of its way to call the Evanuris elven mages, which means that not all elves might have actually wielded magic, even if all of them could use it in some capacity)
...But when Solas slept, spirits showed him wonders he could've never imagined. Then he says "being awake, out of the Fade, became troublesome". So he himself does imply that there WAS some sort of physical manifestation of himself when he was very young (whatever that means for ancient elves).
I think Solas was someone so absorbed with the Fade that he either almost completely forgot of the world outside and whatever physical form he had or maybe even began... well... fading. It might be that at some point he met Mythal during his travels in the Fade and she - seeing the boy's potential - talked him into exploring not just the Fade, but the world.