First let me say how proud I am that you have fostered one of these angels instead of buying one.
I hope you can end up keeping it yourself (if you choose to), since he will bond with you and would be happier there. You obviously have a caring heart to share with him. God Bless you for your efforts!
Can you post a picture of him so we can see? If there are that many feathers missing, it does not sound like a molt to me. He may be picking, or they my be just falling out due to a parsite or ven a nutritional problem. I am glad you are getting him to the vet - that is my suggestion anyhow!
Is he on pellets only, seed only - what is his history?
My parrotlets in general seem to molt 2-3 times a year, but the heaviest molt seems to be in the early Spring (Feb-Mar) -- right before breeding season for them.
I have never had a mite problm with parrotlets, though I have with parakeet, cockatiels, and on the legs of my macaw. I have found nothing anywhere that says whether or not parrotlets even get them. Whatever you do, DON'T
use those mite-protectors that the pet shop has - they are toxic to our parrotlets!
Those little downy feathers are often "sticky" or staticy - can't hep you there.
Though I do hope he leaves them alone so that he has at least a little covering.
In the mean time, make sure you are keeping him warm enough, as he can chill easily with no feathers. Also be sure he is getting the right nutrients in his diet &/or with specific supplements.
Please feel free to message me if you want to talk!
Best of luck with little Azul,