Well, my eperience is that you have to do it slowly but very, very persistently. And it is not a guaranteed success either :-)
In my daughters case, the school would not even think twice about allowing her to grade skip at the beginning of the school year, but they were proposing it themselves by Christmas! (we didn't take it, if you are not familiar with my case:-) My daughter also is not off the charts, but she still manages to be the best student among 6th graders - she is in 5th but subject accelerated to 6th in Math and LA. She studies, does her homework deligently, but school seems to be easy for her. So I know, that even with her (as with my son) it will be a struggle to find challenging materials and proper environment. I take it one year at a time, but trying to think ahead as far as possible. For example, I am now looking for some online classes/programs that she will be able to do during LA next year. If the teacher stays the same, I know her very well and I am sure, that she will let me experiment. We have already talked about it. I am not sure if the school/principal has to be on board for everything? In your case, is that resistance coming from your DS teacher or from the school. I find that sometimes it is easier to talk to the teacher alone.
I totally understand your feeling about regression in the rate of learning due to no challenge. I am actually observing it in my almost 13 year old son. He is capable of so much more than he is being asked to do.
Pursue the school, do everything you can to make sure your child is challenged there. I do not believe in unschooling after school - even though we are supplementing both of our kids education. But I strongly believe that they go to school to learn and not to play! I want my kids to work their little butts off at the school and then come home and have time to play!