Without a picture it is very hard to surmise but I completely understand why you haven't posted one.
As your budgie has been on antibiotics it is a little worrying, is he making a noise when breathing, is his breathing more rapid or laboured, what about his flying does he seem to fly less and pant or seem tired.
Has he been sneezing a lot and lethargic?
Some times budgies that are a bit run down or lacking a vitamin or mineral will have a brown discoloured area above the cere.
try him on some probiotics especially as he has been on antibiotics. Please let us know what the vet says regarding his condition.
As your budgie has been on antibiotics it is a little worrying, is he making a noise when breathing, is his breathing more rapid or laboured, what about his flying does he seem to fly less and pant or seem tired.
Has he been sneezing a lot and lethargic?
Some times budgies that are a bit run down or lacking a vitamin or mineral will have a brown discoloured area above the cere.
try him on some probiotics especially as he has been on antibiotics. Please let us know what the vet says regarding his condition.