It makes sense that they should do this in stages. After all, we're talking about four tiers:
4th grade math
4th grade G/T math
5th grade math
5th grade G/T math
And it might be overwhelming to jump two levels all at once.
However, I think they're missing something important here, which is that if the third tier still is insufficient, keeping your boy out of the last one until December 1 is going to put him pretty far behind the rest of the class. If I were in your shoes, I'd monitor for a bit, and be ready to call a meeting in the next couple weeks if it's apparent that the work is still well beneath him.
