If he knows how to feed himself (because he does eat popcorn), things might go better at school than you think they will, because of peer pressure. My son started preschool at 20 months. I had just brought him home from China, and this program had just started their under-2 class for the first time. I told them I didn't think he'd sit and eat with the rest of the kids. He had obviously been babied in the group foster home (but we are thankful that he was in one that showed him an enormous amount of love, so that didn't bother us), and I didn't think he'd sit and feed himself. And when he went to school with his peers? He sat there and he ate. I was shocked. I didn't get that behavior at home. We had just begun trying to get him to eat with utensils.
Otherwise, at home, I'd stop feeding him and just give him his food. If he's hungry, he'll eventually find a way to get it in his mouth. Give lots of praise when he does it. There will be tantrums; there will be times when he won't eat a thing, but if you give in, he won't do it for you. I suspect he might eat for someone else, but not for a family member. My kids are notorious for being little angels at school--I usually leave parent teacher conferences flabbergasted that my daughter's very serious issues never come up at school... hmmm.... basically, there are certain extreme behaviors that she practically gift-wraps just for family.
There's no telling whether he'll follow his peers or not until you get him in that situation, but you never know.