3 year old hitting at school

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My 3 1/2 year old started full time preschool this year. He is an inquisitive, very active, engineering curious boy. He appears to be quite smart. He doesn't have the greatest language skills, but if he sees something done once, he can do it. He has a 3rd grade older brother at home who is sweet as pie and very accomodating.

My 3 1/2 year old has been hitting at preschool. He's a child that would hit once in a while, but about a month ago had 2 emergency room visits in 3 days (his first and second ever). The pain seemed to flip a switch in him in preschool. He began hitting the children and even the teacher nearly every day. He's been on a behavior chart for 4 1/2 weeks now, earning a star when he gets home if he has a good day and a frown if he hits. If he earns a star, he gets to watch Dora and Diego. If he gets a frown, no t.v. He always cries when he earns a frown.

He started off earning 4 stars, then 3 stars for 2 weeks and this week he earned 1 star. It doesnt' appear to be working. I'm frustrated. He's frustrated. What can we do? We are desparate for some advice.