I completely understand that your son does not raise his hand, there are many valid reasons for that (shy by nature, doesn't want others in the class to know he doesn't understand, embarrassed, etc)
I think you're doing the right thing by taking the iniative to help your son. I also understand your frustration that you feel the teacher will not go above and beyond, but in reality she has done her job and had given you the first steps in guiding you to helping your son. Have you tried using the recommended school websites? Have you asked the teacher specific questions about those websites? Have you looked at your son's math book and become familar with how a particular concept is being taught and reviewed it with your son. Have you asked directly if there are any before/after school programs available where he can get some extra help?
Schedule a conference to meet with the teacher again just before the Christmas break. This will give you a few weeks to work with him, and you'll have a better understanding of his strengths and weaknesses, so you can discuss them and see what steps you need to take next.
There's a good possibility that your son doesn't understand the way the teacher is teaching. Some kids really need more of a visual. Like if they are adding fractions, he may benefit from using measuring cups to see that 1/2 cup + 1/4 cup when poured into a glass measuring cup is actually 3/4.
My girls loved the Jump Start programs.