I'd begin by helping him through his night routine, PJ's, teeth brushing, prayers, story, then lay with him, talk to him about all the things he likes. Then steer the subject to all the things he likes about school. Emphasize on those positive things, talk excitedly about his friends and the fun things they'll do together at school.
The next morning, start with a big hug, at breakfast remind him of a couple of those fun things you talked about, try to get him excited about going in and doing those fun things. Also, talk to his teacher and let her know what's going on. Surly, she/he can say a few encouraging and welcoming words to your son.
Good luck. I know it's hard. Also, say your prayers for your son. That will be your God Send.