Fairli Poh

My grey's name is Basil. I've been keeping him for about a year now.. The last 3 months, his appetite have been really bad. Previously, he was a very greedy bird. Then once he fell sick ( don't know what sickness. Just lethargic and not eating) the doctor gave him 8 jabs of some liver medicine. Now, his appetite is still not improving. I tried giving him parrot favourite foods such as corn on cob but he's scared of anything bigger then a walnut so he didn't touch it. He also throws away his sunflower seeds when I try to use them to train him. Anyone can offer me some advice on whats wrong? I'm very very worried:(