Hi I.,
I am a middle school math teacher, and placement in math classes is always a big issue. I'm glad you are concerned about it and have asked for advice!
It is tough to say exactly what to do, as I have not looked at the placement test and I am not familiar with the math department on campus. To be honest, some math departments are wonderful, progressive, and have figured out the best way to teach math to our kids, while others in California are totally struggling (a whole other issue).
However, I can tell you my 2 cents on your following concerns:
1.) "how he’ll be able to do well when he basically skips the entire 7th grade!" --> 7th grade math is basically a repeat of 6th grade math but at a higher and more rigorous level. If your child did well in 6th grade math (ask his teacher personally about this), then most likely he has most of 7th grade math (Pre-Algebra) under his belt. Pre-Algebra is, in my opinion, a smorgasboard of math concepts that have no connection to one another. Check out cde.ca.gov to see exactly what Pre-Algebra covers.
2.) "It’ll also be bad if he takes the 8th grade but will fall behind and start bringing Cs or Ds." --> Yes, I agree! In my opinion and experience, if your child is being offered to go into 8th grade math (Algebra) while in 7th grade, he should EXCEL beyond the 8th graders in that Algebra class. Go ahead and put him in algebra for the first two or three weeks and see how he does. If he is above average compared to his 8th grade classmates, he is in the right class (again, please be in personal contact with his teacher from day 1). If he struggles and is bringing home poor grades on his first test/quiz, you can ask his counselor to move him back down to Pre-Algebra. Yes, moving him back down could be a ego-damper, but middle school kids are pretty resilient, and if he was truly struggling, he'll be grateful that you moved him back down to Pre-Algebra. It is better to go from a higher class to a lower class than vice versa.
Finally, I cannot overemphasize the need for all parents to be proactive (as you seem to be!) in initiating contact with all your children's teachers. Public school teachers (especially middle and high school) rarely have the time/effort to make contact. However, if they know a parent is willing to speak with them and cares to speak with them, they will actually make more effort to contact you as well should any problems/concerns arise.
Hope this wasn't too much of a read...and good luck! Let me know if you have more questions =)