The challenge of being a "normal" kid when you have something like SPD can be a lot of work. Usually something else will slide (often grades) if a person with SPD doesn't get the accommodations that prevent/lower some of the distractions.

I was a "normal" kid. I figured out a lot of my necessary management techniques on my own to make the outside world bearable. However, my SPD didn't get bad until I was an adult.

If my SPD was as bad as it is now while in K-12, I would have been borderline catatonic in my classes. My last Masters classes in Engineering were pure hell as far as my sensory problems negatively affecting my abilities to pay attention, take exams, and not pass out from the pain. I did collapse regularly (due to too much sensory input) in classes during college and my fellow students often helped me out (threw all their jackets on me when I was cold, carried my books & bags, ignored my carcass on the floor when I asked them to).

Your comment about McDonald's made me shiver. I have food allergies. Very bad food allergies. McDonald's was one of the first places that I realized I had something very wrong with me. McDonald's food (even just thier fountain sodas) caused me to projectile vomit as a kid. As an adult just a bag of McDonald's food passing by me in a hallway is enough for me to end up in the ER with Anaphalyctic Shock. I have a friend that worked for McDonalds. She had to take a shower between working there and seeing me or I would have problems breathing.

If your son avoids something and can't tell you why, assume there is a good reason to it. I didn't know why girl's lacy dresses caused me so much pain, but I refused to wear them. I used all sorts of excuses (except saying that they hurt) when I was a kid to get out of wearing them. I didn't mind wearing dresses made of Cotton, but those weren't easy to find.

Some accommodations by parents make it easier for an individual with SPD to function as close to "normal". Without accommodations, the sensory problems just keep getting worse until a person can become basically non-functional.