We know that he has certain learning problems (with visual/spatial integration, for instance), but he tests at or above grade level on all subjects, though there is a disparity of 18 points between his IQ and performance. The school didn't evaluate him -- we had our insurance do it because they refused.
What do you suggest? We feel it would be counterproductive to have him repeat his grade if he will probably just do the same thing again. It isn't that he can't do the work because of academic difficulties. Clearly, his disorders are getting in the way. My husband and I are college educators. We know enough about this to recognize that he needs help. The question is, how can we make the school help him?
I don't know if your son is in this category, but this situation creates a real problem for gifted kids with disabilities. They may be on grade level, but it's clear that if their disabilities were addressed, they'd probably be doing much better, based on their intellectual potential. They are actually underachieving.
You also said that your son has multiple conditions such as ADHD, ODD, and anxiety. If qualified professionals have diagnosed these, then it's clear that your son has disabilities. As such, his civil rights as a disabled person are protected by federal and state laws. If you feel that the school has not been responsive to his needs, remind them of their obligation under law to make reasonable accommodations for your son. It sounds like they have done this "voluntarily" so they may not have actually violated his rights, but since he's sliding backwards in school, it's time to put some teeth in this intervention by generating a 504 plan. They cannot really refuse you. If they do, then I would suggest that you contact the Office for Civil Rights in your state and ask them to investigate the situation.
Repeating the grade without appropriate interventions in place could result in one more year of the same thing, and that would be a shame. Get a plan in place soon, and see if the school can help your son get back on track before the year ends. If they are slow in responding, you might try getting a tutor or hiring a psychologist to observe your son in the classroom to help you (and the school) decide if he's getting all that he should be getting, and if there's something else going on to account for the downturn in his performance.