Since your son has gotten along well at his previous school, he'll probably do just fine at his new school as well. But he doesn't know that yet, and his fears are not at all unusual. For a child of any age, entering a new school is a big step. Just the fear of the unknown is significant in and of itself but there are also several specific things children worry about. Or he may think he'll embarrass himself by doing something foolish in front of his new class.
Social rejection is a common fear when children enter a new environment.
Your son may worry that he won't have any friends or that the kids will tease him or push him around. Or he might be concerned about whether he'll be able to handle the more demanding tasks of first grade or whether his teacher will be nice to him. The first step in helping your son is simply to encourage him to talk about what he's thinking and feeling about the new school. Ask him what he thinks will be fun and what he thinks will be hard. Then, whatever his worries, reassure him, but don't dismiss his fears.