Sounds like he might be overeating and choking on all the food. Does he have access to food all the time, or just a couple times a day? He might be so franctic for food that he just eats too much at once. You can try giving him very small amounts of food more times a day and see if that helps?
I actually had a clutch of ducklings that, for some reason, would overeat and fill their crops completely up. Then continue to eat until their throats were full and they could no longer breathe and started gasping/choking. Lost one of them since it happened when no one was around, but was able to save two others by suctioning the food out of their mouths/throats when they started to choke. This all happened in the same day (they were only a few days old), so after that we just limited the amount of food they got at each feeding so they couldn't overeat. Never had that problem before or since with ducklings. Such a strange clutch.
I also have one adult Cayuga who used to overeat after it rained and there were just hundreds of snails all over the place. She'd eat so many she'd start to gasp and desperately try to push them down by bobbing her head up and down. Just like your guy. I had to start only letting her out of the pen to forage for a few minutes at a time during "snail season". Now, two years later, there are basically no snails left on our property (due to the ducks), so it's no longer an issue.