That isn't a tumor, it's just a super full crop. As already mentioned, it is likely from the pasty butt...he is constipated and can't get the food in his crop to go down the system. Hold his rear under warm tap water and wash it with hand soap, and then carefully use nail clippers or small scissors to trim away the crusty fluff. Give him a gentle massage around the vent and abdomen to help him move things along...he will probably poop all over the place lol. Then dip his beak in a little spoonful of olive oil and get him to swallow a few mouthfuls, it will help to lubricate all the food in his crop and get it moving through again.