We supplement informally at home, rather than through a formal class, but then I'm a physician and have a good science background; DH loves "pop science," (science concepts, esp. physics, without the math, kind of), which is enough for DS for the time being.

AR and AM will help a lot -- anything self-paced will be a blessing. Since your teacher is okay with you sending in chapter books: We are sending in 4-5th grade level chapter books (on the same topics the class is doing in science) for DS to read during science class, teacher was okay with that. Yours may be too.

My DS6 was a poor candidate for grade-skip (can barely write a sentence and has attention/behavior issues), and we are in-class accelerating in math/reading/science. We wrote out DSs strengths/weaknesses/learning preferences, and also a plan for each subject we wanted acceleration for. We gave those to the teacher, who passed them along to the principal, who was wonderful. We were lucky. I recommend reading "Re-Forming Gifted Education" by Karen Rogers. It helped with the process and gave lots of concrete ideas.

Re the grade-skip -- one grade might not be enough. There's a tool called the Iowa Acceleration Scale, which helps determine whether a skip is appropriate. Also, see "A Nation Deceived," which comes in two volumes; it lays out the evidence for the benefits of grade-skipping:

http://www.accelerationinstitute.org/Nation_Deceived/ND_v1.pdf

http://www.accelerationinstitute.org/Nation_Deceived/ND_v2.pdf

How do you like the Northwestern U. science enrichment? Does DS learn anything he didn't already know? Is it worth the money?

Good luck!