Changing your child’s cohort can be a terrible thing. Friends are moving on and he has to start making friends all over again. There is research that suggests that changes like this mean a child looses 6 months of education, while they are learning to fit into a new cohort. It is also possible that symptoms will get worse, because he already knows the material and is bored. I know you say no work has been done, but it may be in his head and he doesn’t test well. Please research this a lot and ask your child a lot of questions about what happened this school year. The stigma of being held back is not something a child gets over.
I think the bigger question to answer is, how did your child go an entire school year without doing any work? The IEP is obviously failing. If there is a BIP that is obviously failing as well. The school and/or teachers seem to be failing. Is there more here that is undiagnosed? Does medication need to be re-evaluated? I would try anything to keep from holding a child back a grade, especially in elementary school where the entire building worth of kids knows he should be in a different grade.