Read your entry with interest and wonder if my humble suggestions may help :
1) Can you request to sit in temporarily when he has his lessons until he settles down in class ?
2) Talk to the parent of the kid who slapped your son. Not to confront the matter in hostility but arrange for your son and the so-called bully to have some playtime together. This gives your son and the other kid a chance to bond, in a way your child actually confronts his 'fear' and slowly the fear diminishes, hopefully.
3) Discuss with his teacher about his eating habits. If it is possible for you to bring some of his fav food/utensils from home to be used in school, in order to entice him. And for the issue on food, ask the teacher if it is possible to introduce the food that your son dislikes slowly. Who knows in time to come, he'll learn to enjoy the food from school.
The important thing is, you shouldn't get weakened down by your kid's cries and tantrums. Good luck!