In my experience, a "diagnosis" determined by the SD doesn't/shouldn't drive the IEP. What drives the IEP is what services your child needs to receive FAPE in LRE. So whatever the educators want to call it, in my book that's great, as long as it gets the educational services. My oldest was "developmental delay" until he hit first grade. Then he morphed into orthopedic impairment and visual impairment (he's got optic nerve atrophy and cortical visual impairment on top of his cerebral palsy). When we moved to IL, he suddenly became "other health impairment". Whatever. It's bureaucratic bologna designed to get the most funding - because his disability is very severe, he's a "low incidence" kid and therefore I guess the district gets extra $$$ because of how they classify him.
The educational classification does *not* drive what services your son gets via the IEP. Your son's needs drive that.
In terms of "real life" diagnosis and treatment, I would stick with- neuropsychs, developmental peds, psychiatrists, and other MDs for the medical diagnosis. Remember that the IEP team only has you and educators on it - they are not qualified to make a medical diagnosis.