I don't know if this will help because my tiel seems to be a little cooky compared to other birds, but maybe you can try finding something that gets his attention. Like a colorful toy or some colorful blankets.
My tiel, whenever you wiggle around a blanket, sheet, towel, shirt, whatever, he goes CRAZY for it. Starts singing all these tunes, gives me his heart wings and runs around in circles. It's pretty funny, but I've never seen another bird act that way with towels. Maybe you could give this a try to get your tiel to sing again. Obviously be weary of scaring him because some birds are scared of towels, but introduce him to it.