Half day is easier for kids to manage, but a lot depends on the child and program. How structured is the program? How much would DS be expected to sit still, pay attention, follow directions? How much worksheets or other type work is being done? What about the other kids? Are they typically developing?
It's very hard at times to predict how a child will react until they are placed in that environment/atmosphere, but an overly structured program may backfire.
Of course, another issue is that your DS could go to K only to find out that they don't really 'learn' in K. Many K programs followed a structured curriculum today and are focused on the basics, which your DS may have already mastered. Additionally, your son may have higher expectations/abilities/skills than a K program could deliver. He may find the other kindergarteners not up to his level; and it's hard to predict what he may do or how he may react.
Sensory issues are not nonsense. They're very real. The psychologist is not being professional, imo, there; she's stating her opinion, not fact. The psychologist should let the OT speak about sensory issues and respect the field/profession.
Pediatrician is correct that attentional issues are normal for young highly intelligent boys. There's a lot of misdiagnosis of ADD/ADHD with eg/pg boys, both over and under diagnosis.
If the K program is a half day and not overly structured, I'd agree with the pediatrician that you might try it out - provided you're easily able to withdraw if things don't work out as expected.
