J.F.
I am a tutor and also have ran a company similar to Sylvan. I would ask your local school for a referral to a tutor they are fimilar with. That person will be able to give your child the one-on-one attention he needs, especially if he needs help with current classwork. It will be less money and possibly hassle. (I drive to the student's home or meet them at the library.) Most private tutors charge per hour ($25 - $40 an hour). I work with the kids either 1 - 3 hours a week depending on their needs. You can also look at www.care.com to find a tutor in your area. Make sure they have their degree in education and have a background check (care.com does this).
Sylvan will work on the skills your son is lacking, but not current classroom work. He will be placed in a group of up to 3 kids and be asked to attend 3-5 hours a week. They charge between $40 - $50 an hour and pressure parents to pay up front for several hours at a time.
Sylvan and Kumon are great for when you can't find a tutor that knows what they are doing or when your child is 2 years or more behind. They use a lot of drill and memorization which is fine for most kids. If you have a kid that needs to draw, imitate or manipulate objects to understand it, it is not the best environment for them. The tutoring center is also great over the summer when kids get a chance to catch up to their peers and not worry about daily homework. You can also find a private tutor to do the same IF they have curriculum that can be tailored to his needs.
Hope this was helpful! Please note I am not against Sylvan or other tutoring companies. They are a great resource for when you can't find the perfect one-on-one tutor.