With you there, let your son meet the teachers, look around the rooms, and try out one of the desks. With your son listening, ask the teachers questions about the work in second grade, what the daily schedule is like, and what the rules are. Your son will see that second grade is not very different from first. Make sure that he hears that the teachers still take good care of second-graders and that he will be ready to do the work that he is given.
You can also ask your son's first-grade teacher to talk with you and your son about second grade. She will most likely reassure him that he is ready and the work will not be too hard, and that he will still be able to see her around the school.
Make sure you talk about the second grade with your son this summer. Also, alert the school counselor to the problem so that she will be ready to assist him in the fall if necessary.