I agree that you should take him to another vet if he gets worse, and if in your opinion this
cyst/
abscess has not been cleaned, drained out, or attended to properly.
Although the antibiotics should usually help, an antiseptic wash is also usually recommended after the pus has been drained out. And if it is an abscess, it is wise to keep it open, keep it bathed with warm water and antiseptic until all the pus is gone, and it shows clean pink skin.