Mowgli is such a cutie pie - we hope you have a diagnosis soon for him.
Just to reassure you about blind cats (and you have received some excellent info so far from PT folk who have blind or sight challenged kitties) this is our Pippin:



He was blind from birth (no eyes) and while I think he enjoys being part of the Tribe of 7, he could easily be happy as a single cat - he is especially attached to CatDad- he is VERY independent, goes where he pleases, chases and plays with his toy mice and is an exceptional track ball player. The photo is of him on the kitchen counter - he climbs like a champ and only occasionally gets stuck and meows for an assist in getting down. We've put in some adaptive ramps up for the cat trees and other fun places he likes to climb and other than that, he requires not special treatment - just lots of love, crunchie treats and preferred spots on the couch, the bed and wherever else he can muscle out one of the other boys. Pippin uses his hearing almost exclusively to navigate around the house and he has memorized where the rugs are on the floors and corners of the walls to help him figure out where he is .
To tell you the truth, we forget most of the time that he can't see. I think if you are considering another cat, I wouldn't do it just because Mowgli may not have his sight some day. I would consider all the other normal things you think about when deciding to get another pet. Will they fit in to your lifestyle right now, will they get along with your Mowgli, can you afford it now....etc. Mowgli is going to exhibit normal cat behaviors and he will adapt to life without full sight - Pippin gets into play fights and sometimes real arguments with his brothers - he is a force to be reckoned with when it comes to the most comfortable spot on the couch or bed and is just a normal cat in every other way except for his sight.

Good luck and we do are hoping all works out for you and your little Mowgli.