Some people have gluten intolerance, which means they can tolerate some amount of gluten (usually), but only a limited amount which might be very small. Coeliac though is different. It means that no gluten whatsoever can be tolerated and eating it will keep doing damage and will stop nutrients being absorbed. If your DS has coeliac and is still having gluten, giving him iron is not going to make much difference as he won't absorb it. He must stop all gluten now and forever. I find it very strange that doctor wouldn't tell you that - perhaps it's time for a new doctor. And definitely a dietician - this is really important to make sure you cut all the right things out, and still leave a nutritious, balanced diet.

An elimination diet is hard at first, but it gets easier. Once that's done, then you can see what happens to the iron levels and you might find supplementation isn't necessary anymore. It will probably be necessary for a little while until the levels have a chance to get to normal, but then with any luck he'll be fine.