We are also a middle income family and have hired advanced high school students as tutors for our kids rather than professional tutors when they were younger.
ALEX is a very inexpensive math program that might work for your son.
I have no first hand knowledge of Kumon, but my friend has used it with her young daughter and has been very pleased. She credits it in helping get her daughter early entranced to 1st grade.
Are you familiar with the Mathnasium franchise? I think the cost is approximately $200 per month for after-school group tutoring. It might be a good short term solution.
