I don't know how SPD would impact the WISC, other than if he's having a tough time focusing due to sensory distractions his scores might be artificially low.
Did the tester mention why coding was "spoiled"? I wonder if the fine motor issues he's been referred to for OT didn't contribute to the lower coding score, and that's something that will come up after he's gone through OT?
I also wanted to let you know that my dd10 had sensory challenges when she was younger, and a year of sensory OT while she was in kindergarten helped her SO much! OTOH, it didn't make a significant difference in her IQ scores, but it made a big difference in her ability to navigate in the world and focus etc. and it was definitely worth pursuing.
Best wishes,
polarbear