I cannot imagine, having broken a cheekbone which must be agony, that it's easy to stick a bit of plastic on it and then think, oh no bother now, can't wait for my next header!
Your head tells you it will be fine as that's what the doctors said but it doesn't stop you freaking out every time a ball's kicked your way, and I would think it would take a couple of weeks of a gradual increase in training for him to feel psychologically ready to head balls/get involved in tackles etc.