It really sounds like the school - or children in the school are the problem. If so, I doubt that he will have these problems in the new preschool. But I would certainly try and make the new school familiar to him with visits before he starts there.
And, I would also try and nicely ask his teacher what seems to start his actions. Does this happen during breaks, recess or during class time? He throws tantrums when he has had to sit in time away. Why did he have to sit in time away? Its kind of hard to tell if the other kid is a bully or reacting to what your son is doing in class. So its kind of important to figure out (if you can) what the triggers are for his melt downs.
humm I can see its difficult but by your description of your childs behavior, he needs protecting and you really should speak to the director about the matter .or remove him to another school .. A dilemma,dont we hate them, but your son comes first . This is most certainly in my opinion , the problem .
He is an only child and he is very well behaved at home, his aggression has only been occuring at school. Dealing with the other child may be a little difficult since of all children in the entire center it is the directors son (also my boss) so I am unsure how to approach the situation. The teachers know this particular child is aggressive towards the other children including my son, but I don't know if he is being singled out.
From what you say especially regarding another child calling him unkind names I would say thats why he doesnt want to go ..Have you told the teacher about the other child ,it may not all be about your son he may have encountered some bullying and he has been soley picked on ...Is he like this at home , has he any siblings ?