Little glutton bit more than he could handle.
Relax, I really don't think a fish can choke - any food that got into the gill chamber would have been flushed out immediately. As long as the gill cover is moving, he's breathing OK.
The big lump of food is probably stuck in his esophogus and it may be causing some discomfort - hence the thrashing about in an attempt to make it go down. It should go down eventually. Tomorrow, I would offer him a partially boiled pea with the skin removed. That will prevent any intestinal blockage.