That is definitely baby bird behavior. Even if he isn't a baby, it might feel good and putting pressure on the right part of his beak is clearly stimulating a feeding response. If he's recently weaned, I'd be watching his weight to make sure he isn't getting super hungry. Actually, I'd be watching his weight once a week anyway just for health monitoring, but daily if he's younger. If he's several months post weaning, then it probably just feels good to him. I wouldn't worry about it if he's old enough to be very well weaned but I also would not encourage it.