Before I get into the specifics, I feel like I should mention that your character is AWFULLY cute <3
His appearance.
As I've already sort of mentioned, this is a very cute character. The color scheme is very consistent and flows nicely, without contrasting too much or being too oversaturated or undersaturated. I enjoy the idea of a sort of fluffier-than-usual character because I like those type of quirks, but make sure to work this into their story. Major differences have an affect on how a character will be perceived in the universe they reside in, and having an excess of fur also means having some physical disadvantages because he might be likely to overheat easily in some scenarios (especially given the amount of clothing he needs to wear to protect from sunlight). My only qualm, and this is purely me being nit-picky, has to do with your burn marks. They just don't look like how a burn scar would look on an animal. You can express this in a more PG way by giving him a very light patch where no fur grows, since that is how burns work in the real world. Since fur density isn't typically apparent on MLP pony faces, you should be able to get away with just giving him a jagged little marking, but the fade out you have wouldn't be realistic and it definitely would not be dark. Burns are only dark at first before the skin heals over.
Note- This is the usual case and is typically what is best to follow for simplified drawings such as that in mlp style.
Burn scars on animals [TRIGGER WARNING FOR SOME PEOPLE], I tried to keep it relatively safe to look at and not too obscene:
His cutiemark.
While I can assume that it wouldn't be unrealistic for a mlp character to not find his talent early on like most ponies, a mlp adult character without a cutiemark has never actually been seen in the show to my knowledge. Strangely, at 16 he would technically be an adult given that Twilight and her friends are all marked to be around 15+ at the beginning of the show. Cutiemarks are a very important part of this society, and I can imagine him not having one being very hard on him emotionally, especially given his other trauma and trials.
His personality and bio.
I really enjoy the personality and cute little flaws you've given him. I think that the blindness is a little bit of a stretch, though it is acceptable if we are to assume this is something that was developed at a young age. I say this because adult eyes are very developed, and cave blindness is very unlikely, if not impossible, to occur in adults. However, for young children light deprivation can cause significant sight loss because their eyes are still developing. If a new born baby is not allowed to see for several months to years after birth or during the first to second year after birth, it can end up with a serious visual handicap, or may be even completely blind, despite having completely normal eyes. This happens because the visual cortex in the brain fails to develop in the absence of a visual input. This means that your pony would have to have been in near complete darkness for a VERY long time, directly after birth, so just keep that in mind. That being said, fur on animals is meant to protect from the elements, including cold, water, and even sun. As much as I like his fluff, I do think that it is completely incompatible with his apparent allergy to direct sunlight, because his dense fur would prevent the UV rays from making much contact with his bare skin. When animals take allergy tests, their fur needs to be shaved down so that the skin can be tested, because hair is already "dead" after leaving the skin and follicle.
I will be getting to your critique shortly within this evening or tomorrow c: Your review takes only a tad bit more work because I have to imagine the visuals, rather than having them right there infront of me. It isn't a problem at all, it just means I have to apply particular attention to your descriptions.