Mmm. How did they determine that your DS is 99th percentile for maths, and how does the teacher plan to assess in order to check that he's challenged? E.g., do they already have, or will they soon have, evidence they believe that he already knows the 4th grade curriculum? If not, an obvious first step is to ask that they assess him at that level. If they do, then surely they must be prepared to teach them 5th grade material.
I think I wouldn't knock the large class math olympiad and chess class - I could easily believe that would be more beneficial for your son than a standard 5th grade class, especially if he already knows most of the 5th grade material too. He will probably benefit more from being given problems than from being fed solutions, so provided it isn't just that the teacher stands at the front and explains how to do each question until everyone in the class understands it, should be good! I'd focus attention on the other classes, where he isn't likely to learn anything. Perhaps they could give an end-of-4th grade assessment to the whole of the top group, and then let those who do well on it be a subgroup?
How is he at self-directed learning? Would he go for being allowed to work on Olympiad problems on his own, or maybe in collaboration/competition with any other children who also don't need 4th grade maths, during the other maths classes? I would tend to think that being ready for next year at a school where maths is one year ahead of age isn't likely to be an issue if he's most of the way through 5th grade maths already; surely he could do anything missing over next summer, if need be. Seems to me that what you need is a plan to get his school time be spent on something valuable, regardless of what material it covers.
