Thank you, everyone for the replies. They are very helpful.
FWIW we're trying something similar with my DS6. So far (4 days in!), the main problem is that he has no skills to get himself unstuck or go on, when he reaches something he doesn't know how to do. So at that point, he just stops, and stares at the question he can't do until told it's time for a different subject. Argh :-) Yours is a bit older though, and even with mine, I'm hoping he can be induced to learn not to do that!
ColinsMum, I anticipate a similar problem if this is what we end up doing with our son. What I will probably do to entice him to work through the problems is tell him, "OK, if you can't figure out these, just move on to your long division worksheet. Finish one or the other - your choice." I think the prospect of a page full of long division will motivate him to spend a few minutes contemplating an algebra problem! Of course, I will need to make sure that the more advanced problems are do-able and give him a little wiggle room to do them partially.
Bh14, Inky, if I am following you it seems that you are both suggesting I be pretty up front and just ask for the 5th grade placement outright. I think I've been hesitant to broach this because I feel like on some level it's insulting to the teacher. As though I am suggesting that it's obvious she can't challenge him, even though she's said she is going to. OTOH, he's been in school FOUR WEEKS and has yet to see a math problem beyond single digit muliplication (with the exception of the Math Olympiad problems, but that is done outside of the math curriculum).
Inky, if you get a chance can you elaborate on your reason for suggesting I be up front about the charter school? I'm little hesitant to do that. ALthough it might alleviate concerns about what will happen after he finishes 5th grade math ( the school only goes through 5th grade).
Yesterday afternoon my son told me that they were given a math test instead of having math class. I had seen something about testing this week, but when it didn't happen Monday I figured it was only for the kids who weren't already placed. The test was over an hour long and my son said the problems were like the ones I made him do over the weekend (I had him review the 4th grade end of year exam since he hasn't done any math work in a month). So he should have done very well, and if he didn't then maybe I have assessed him incorrectly. I anticipate getting the results on Tuesday when the weekly folder comes home. So at this point I am planning on waiting for that information and checking the work he brings home between now and then. I also need to try and find out if the fifth graders even do math at the same time. If they don't there could be all kinds of legitimate objections on logistical grounds.