5 year old boy suspended 4 times in one term

My son has been suspended 4 times in one single school term. The problem is his hitting other students and teachers. His English is not good and he lacks of  social skills. He is able to get along will adults very well, but not with the children of his own age.

The school asked us to pick up him 2 hours earlier than normal, saying that it is to prevent him from hitting other children. I don't understand and we refused to do so. So the school suspended my son 5 days after his hitting again.

Whenever Ｉask my son what happened at school, he says 'I don't know'. so I am not able to know the detail of school. And it looks like the school didn't know why my son hit other children. My son wouldn't do Time-Out at school, and whenever he doesn't do it, he will get suspension. The previous suspension period was 1 day, 2 days, and 2 days.

I am sick of this, I don't think the school has done its best. The behavior teacher only knows to provide us with some behavior chart - not working.

I know there must be something that we, as parents can do to help my son. Please help me.

[Sandman2]

   Yes, there is probably things that you and the school can do to help your son.  At his age I am guessing that the hitting is out of frustration, but I/we need a bit more information.
    Is this a public school? - kindergarten or pre K and when is his birthday?
    How is he doing academically?  Maybe not well if his English is not good, but how about the other subjects?
    Has he been in the school all year?
    Finally, do you have any problems at home with him losing his temper?
  Oh, and what is going on at school the last two hours of the day?