In your situation, I would aim for a pathway that gets him into the 4/5 compacted class, assuming this is a gifted service, even if it sounds like it takes him to the content a little later. If it's compacted to honor the needs of gifted learners, that is, reducing repetition and minimizing the drills, then this will be a better fit to both his age and learning needs, as well as bringing him into an environment of gifted kids. In contrast, the current 4th grade class has those kids missing from the classroom, the content will move more slowly, and will likely involve more drill & homework.
I have requested that math accelerations be based on curriculum-based testing. The MAP scores are something that motivate the discussion of an acceleration, but the level would be better decided by the grade level on which he can show proficiency in the curriculum. This means he can demonstrate not only mastery of the concepts, but can perform then in the context of how the school teaches it.
DS went from K->2 for the last 12 weeks of K based off of standardized testing and end-of-1st semester curriculum-based test of 2nd grade math. He then skipped 1st grade, but stayed in 2nd grade math for the first quarter with the agreement to re-evaluate at the end of the quarter. After those 9 weeks, his teacher started compacting 2/3 for him, and we formally accelerated again into a 4/5 compacted class as a 3rd grader once he scored highly on the end of 3rd curriculum-based test.
This approach of effectively skipping 1 year, make a plan for when to re-evaluate, and skip again, worked really well. We were also able to time it to put him into the gifted cluster, which meant that he was getting the pace and depth that he needed with peers that weren't as old as they would have been otherwise. He's now doing another compaction program for middle school math. Pacing & "peer" group (2 years older) are really important to keeping him engaged.
The next thing to consider: Scheduling is the bane of our existence. I would ask the school develop a plan for each remaining year of elementary school so that he can both access his math education and the rest of his education. I have found that beginning the discussion of scheduling by January for the subsequent year is necessary to achieve anything close to workable.
