it is difficult when your child has been with you to let them go, even if your child is tearful, I can guarantee that within minutes of you leaving he will be ok and it is good for your child to be mixing with others.
when my daughter started school she cried every morning for about a week when I left her, but after going home I phoned the school to check and she had settled down and was playing happily with the other children.
dont give up, you will probably be more upset than the child, it's normal :)