Hi there! I am a 6th grade teacher so I might be able to help :) Check into what textbook they will be using. We just got new textbooks this past year and they have an online textbook with quizzes, games, and lessons. I recommended that for a student who struggled a little bit this year. I also liked a website I found, www.ixl.com. It has some free access but you can also become a member, not sure the price, and have reports printed out, etc. It goes by Illinois learning standards and grade level and it was pretty kid friendly. Otherwise, you can also spend some time online looking up the different topics they will be studying in 6th grade, they have tons of games, and actual presentations. One last thing, the local library should have a copy of the school's textbook, you can always take a look I am sure. Oh and one more thing, the school's website might have access to the 6th grade curriculum map which should show the topics/chapter covered in 6th grade :)
Be careful with the testing thing, schools don't like to automatically jump to that.. if we tested every kid when they suddenly struggled our psychologist would be pretty busy and we'd have lots of kids tested unnecessarily. There is a bell curve for a reason! Fact of the matter is some kids and people are just not good at math. BUT, you are correct in being an advocate for you son :) Plus schools are moving to a new gen ed initiative to provide interventions for struggling students and a new model to diagnose students with learning disabilities.
Hope that was helpful!