I could mention he wears a 7 hat. My head is bigger than that. I think 7 3/8.
It sounds like you are very concerned. Since you have the headaches and sinus problems, do see a doctor. Maybe an MRi is in order, but this may be just normal for you. Are you different from family members? The headaches may be unrelated, so prepare yourself for that.
My BF has a small head also, and he keeps his hair short. It looks okay until he tries on hats. Then he tries them all on just to hear me laugh. I am so glad he is not self-conscious about it. He is very good looking. With longer hair you'd never know. I'm sure if he were overweight it would be more noticeable.
In case you have milk allergies, you might want to stop dairy and see if it helps your sinuses and headaches.