And considering that Inquisition keeps engaging with stronger enemies as time progresses AND has reputation to uphold, I think it would be strange if he kept running in basic gear... Ironically, at some point he'd start sticking out like a sore thumb if he wore it.
"Say, who is this elf running around in that shaggy green vest among all those soldiers in lustrous plates and silks?" xD
That's exactly how he ends up looking in my playthroughs on purpose!
I would use the Winter Palace Formalwear schematic and craft the same uniform for everyone except Solas. So you see a party walking in the field, everyone in the official Inquisition Elite Forces uniform, except for the weird elven mage.
I like to imagine him as the type of guy who would stubbornly keep wearing his favorite clothes no matter how worn out or inappropriate they are.
Inquisitor: Solas, your outfit... Did you make it yourself?
Solas: Yes.
Inquisitor: The craftsmanship is... unconventional. Also, your coat has seen better days. We need to get you better gear.
Solas: I have specifically designed my gear for optimal durability and channeling the raw energy of the fade.
Inquisitor: Well, this might be high fashion in the fade, but it's inappropriate for a member of the Inquisition. You need to wear the uniform.
Solas: It is also incredibly snuggly.
Inquisitor: *sigh* Okay, how about you wear these ancient elven robes... You know, in the name of Arlathan?
Solas: Hm... I will have to consider.