There is a distinct possibility that the cat was not properly socialized as a kitten. They need to be handled by humans VERY early. You may never make him a social butterfly. I have a couple of "no-see-ums" myself. There is no way that these cats are going to allow other humans to see them. I do not know why, but I accept them as they are. If your cat is good with you, and the family he lives with, and is not suffering in any other way, you just might have to let him hide when there is company.
Sorry I do not have more suggestions, but if you do find an answer, I may need to listen to that advice myself.