That is so frustrating....I would stick with the GT Math, even if he wants to try every other day for a few weeks. If the class is good, he may be willing to give up lunch and recess. If the class is sort of good, every other day may be plenty. I figure if he's willing to give it a try every other day, why not?
Golly - too bad that they moved it - so frustrating...but if the teacher sees that he really needs it and 'gets' him, she'll push harder for him next year, right?
Keep the long term goals in mind.
And he may have all sorts of feelings about being with older kids, being actually learning for the first time...trick is to keep him 'in the game' for a while so reality can overshadow those feelings.
Good luck,
Grinity