That's frustrating! What kind of cup does he drink out of? Straw cups are better (from what I've read) as they help the child use and strengthen the muscles they need to swallow. Do you think something like that could be the issue...maybe he doesn't know how to chew and swallow properly?? I've never had a child hold food in their mouth but maybe someone who has could offer suggestions!
I would give C very very small portions and if he finishes he can have more. Maybe it overwhelms him?
We don't have treats around here very often so I don't worry about whether they ate or not to get their dessert. Maybe decrease the treats? When we do have a treat, I do try to make sure they've ate a good portion of their lunch though. If we do have a treat, it's usually at afternoon snack....I don't want them all sugared up LOL
IMO...I've had better experiences by just giving the child the same food over and over versus making them eat it. They eventually will taste it when they realize you won't quit making it...or they get hungry enough lol. Like lee bee said, you don't want to get into power struggles. Remember there is more to eating healthy than just daily fruit and veggies.