My 3 year old son has issues trusting his peers?

When my son Samson was 16 months old he was picked on by one of his peers. He was pushed, bitten, scratched, hit, you name it. This happened over a year. Now he is 3 years old and goes to preschool. Toward the beginning of the year Samson was coming home with scratches and was having issues with his teacher. He won't engage his peers or his teacher. He talk and plays with the T.A. but he won't play or talk with his peers. What should I do?? Can anyone suggest any resources? A good book, website, hotline? Anything?