1. Stick with the 2-year acceleration plan, which DS and the administration would be happy with.
I would choose this option - primarily because your ds is happy with it, and also because the GT teacher seems to think it's a reasonably good fit.
While he would most likely do a-ok with another acceleration, I also don't think it's going to hold him back in anyway at this point in school not to push for another grade-level subject acceleration in math. If you find that once he hits middle school he would benefit from another bump up in math, you might find it easier to make the bump by sending him to a course in the summer or he might be more willing to take an online course.
Best wishes,
polarbear
