I am so sorry to hear about Ernie.
I hope he feels better soon and that the vet can help. Until someone with more experience weighs in, the best advice I can give is to provide some Guardian Angel for your budgie. https://www.talkbudgies.com/your-budg...te-recipe.html
I do think it is good that you are keeping him warm enough and minimizing drafts.
Sending good vibes your way.