Is there a district supervisor who oversees gifted services for all schools? It might be worth putting a call in to that person and ask for help/suggestions. My guess is your ds probably isn't the first student who's run into this issue - no matter how deer-in-the-headlights the staff looked at your meeting... unless he was the first student ever accelerated in math at his previous school.
How do you feel about asking for a grade skip? Or about the other options you mentioned - requesting he be in a 4th grade classroom part of the day, or partial 1/2 day at school and 1/2 day alternative (homeschool, CTD, etc)? I'd think through what you would prefer and what you think would be the best alternative for your ds and that will give you most of what you need to answer your question re what differentiation to ask for.
Best wishes,
polarbear
