It sounds tough for all of you. I totally agree that it's normal to be sooo picky at that age, but to actually refuse food all day is a bit more intense and suggests he cares about this a lot. There are feeding therapists and group programs. It could be a combination of things, where he associates some food with pain or has motor issues from the celiac. But they can be tough habits. I'd try the suggestions above but if you have insurance that covers it or the funds, I don't think it would hurt to consider feeding therapy.
My DS has OT for motor issues and has needed help just eating "hard" foods. I think without his OT honestly telling me that if he spits out meat to have him pop it back in , I would NEVER have done that, but you know it worked and he needed more than the avg. kid who just needs to be exposed.