Weighing in as a teacher here. If you're seeing frustrations and difficulties with math, you are wise to go ahead and address it. Math builds year after year, and your son will benefit from a strong foundation. It will give him more confidence entering third grade (or is he a rising second grader?).
I would echo the PP recommendations of Hill Tutoring or an individual teacher. The Hill Center does an excellent job helping children with their academic needs, and I've heard some good things about the tutoring division too. As a teacher myself, I've worked with a number of students from my school each summer. I have the advantage of knowing the expectations at the school, and I definitely charge less that most tutoring agencies.
One additional resource that might be helpful:
Tenmarks is an online math practice site that has a free summer program that we recommend for our students. It starts with a diagnostic test, then assigns practice exercises mixed with games to keep kids engaged. I would not suggest it as a replacement for a tutor, but it could be a great supplement.