Have you thought about Kumon or Sylvan?
KUMON Pros: not terribly expensive, no testing fees, and homework that is done daily (vs. only work done in "class"...helps keep skill level up) They'll start your son a little below his natural abilities to give him confidence then move him up the ladder.
(possible) KUMON Cons: Check to see if Kumon is the right fit...my 3rd grader is working through the basics right now (addition, subtraction, multiplication, division) to increase his speed. I don't remember what is covered in 7th grade, but if your sons basic skills are lacking this will definitely impact his ability to do higher level math (algebra, geometry, etc.) The only other possible con I see is that you sign up for 6 months when you start, so you'd be continuing on into Fall. Since this program is designed to help with skills, not homework, you'll want to check to see if this will be a good fit.
As far as Sylvan is concerned, their program is very sound, they hire qualified teachers, and they know what they are doing. The program is more expensive overall (testing is at a fee and the weekly classes are more expensive) but it might be a better fit for your son's needs, as they DO help with homework during the school year, if you like, and they don't have a set period of time you must stay in the program.
Good luck! I know this is frustrating, but you are getting a head start on this!