Hello,
Please stop asking him what's wrong. It implies something's wrong and that gives him more to worry about.
If it's medical, then the doctor will know what's wrong.
If it's nervousness at the thought of separation, a very common situation, asking him what's wrong makes it much worse. Just keep in touch with his teachers to make sure the days are going well, don't ask what's wrong, don't "prepare" him for going to school by talking about it, keep it low-key. I know this is not fun, but you will get through it. Really :)
P.S. I am concerned about your husband getting mad. This sends signals to your son that he is doing something wrong. He is not. Ask your husband to think of it this way "It's not like he gets up every morning and thinks to himself, let's see, should I have a pleasant, good-feeling morning or should I vomit and have diahrea?" It is very importnat that your husband talk to the doctor with you and be involved in the solution.