ebeth,
In the middle school my kids have gone to, out 7th grade science is good, but not that tough - I really believe an advanced 8 or 9 yr old could do it.  I'd be surprised if my sons take more than a few lines of cryptic notes, and really that's only if it's required.  If you decide to proceed with the acceleration-- remember, you can reverse it if it isn't working.  
It's such a shame that time will be spent traveling to/from the other school, especially in today's age of technology. What does he miss during the traveling? I wonder if you could visit the class with your son and see what it's like (and if it's worth the trouble, stigma, etc.) 
Everywhere I look there are virtual labs available free online, free podcasts of science info, tons and tons of stuff out there that no classroom teacher has time to access even a fraction of it.  Yesterday I looked at the NASA website--lots of stuff.  Our state Dept of Health has a lot, too.  EPA website is loaded.  Cogito.org. Videos--Discovery channel, planet earth BBC series, our library has many.  Science experiment books.  I guess what I'm trying to say is, isn't there a way to give him time, opportunity and a little proctoring to do some advanced independent work?  So many things to explore, so little time.  
Mind you, I am rarely one who would shy away from an offered acceleration.  Maybe your middle school and later your high school are pleasant, civilized environments...But! I just walked through our high school and got caught in the hall when the bell rang.  It was Crazy.  Large people, some scary and weird, and so many kids.  I actually thought about my DS10 and how I couldn't imagine him going there for math class (which is what I thought about this year).  Too frightening for me & I'm Old.