If there are environmental factors you can control, do. Dimmer switches on lights, cut the tags out of clothes, let him wear his socks inside out to avoid the seams. Make his life as cozy as possible and cut out the harsh shades and lights.
A little boy I babysat for would sneeze as soon as the sun hit him. He’d be asleep in his carseat and if we turned and the sun hit him? Achoo! His brother needed all the tags out of his shirts, all his socks inside out.
My daughter hated buttons. No idea why as a three year old, and no idea why as a 25 year old. She preferred soft knits like t-shirt dresses to crisp cotton with buttons or even jeans with buttons/snaps. Whenever I could I let her totally irrational fear of buttons rule (her school uniform had 2 tiny buttons on the polo shirt and 2 decorative buttons on the skort; she had to deal). Most of her clothing was loose without belts, buttons, decorations.