If he's already got an IEP and receiving the appropriate services I'm not sure it matters much to have the official diagnosis.
In some cases, I'd agree with this. Here I wouldn't agree, because understanding the correct diagnosis is needed to understand what's needed long-term for accommodations and remediation. If this is SPD, there's a chance that OT is going to remediate the issues with handwriting fully and accommodations aren't going to be needed long-term. If it's dysgraphia, chances are good this child will need to rely on keyboarding or voice-to-text for the majority of written work moving forward in school and life, and getting a head start on that is important.
Having a correct diagnosis and understanding potential symptoms/challenges associated with it can also help you ferret out information from a younger child - for instance, hand and wrist pain are sometimes associated with dysgraphia. We didn't have a clue that our ds' wrist hurt when he wrote, but once we knew to ask about it, he told us absolutely it hurt! It's just that being only 8 years old, he didn't anticipate that we, as his parents, didn't already know that. My dd otoh, had some fairly severe sensory challenges and went through sensory OT as a child. I don't remember pain ever being mentioned as a potential symptom - so if our ds had been diagnosed with SPD and not with dysgraphia, we never would have asked about the hand pain.
Having a correct diagnosis can also be important for helping a child understand what's going on with themselves.
On the flip side, I can't really see a situation where having a diagnosis is going to hurt or cause harm in any way, so if there's a question I can't imagine it wouldn't be a good thing to move forward and really get to the root of a diagnosis.
polarbear
ps - fwiw, my ds has some sensory challenges too which I understand from his neuropsych are part and parcel of his overall diagnosis (Developmental Coordination Disorder) - I don't think it's unusual to see both dysgraphia and SPD in the same child.
