It looks like his mouth is centered ok but it does look like it is extended a quarter of an inch or so. Does he still eat? If so, then I wouldn't worry too bad but it might be misaligned. Has he liplocked with any other fish?
he hasn't eat anythign for the past 4 days. I try to push it back and it go back in a little bit but mins later, it just pop right out like that. Could it be the "cotton mouth" disease?