I was wondering what the source for this information is. As far as I know the film doesn't specify that the comatose patient on Muir Island is a relative of the Professor's; was it in the novelisation? I'm sure there are many popular fan-based speculations, one of which for me has been that it is in fact the movie-verse Proteus, but only a genetically cloned "son" rather than an actual biological one. Genetic engineering has been an undertone throughout the X-Men film franchise, and it wouldn't come as a surprise if Charles had some sort of body-double which has an (uncanny :P) physical similarity to him but lacks a functioning brain; perhaps he and Moira cloned that specifically to test the limits of telepathy purely for scientific reasons? Maybe it's something that Charles made when he was McAvoy! Haha. Either way, come DoFP, we'll surely find out!
Another note: I love how in X3 we have the body-jump, and side by side we see cameos of Psylocke and Juggernaut (lets face it, that WAS a cameo, albiet a long one); the first shares the body-jump "backstory" with movie-verse Charles whereas the second shares the brother-relationship "backstory" with comic-verse Charles. I don't even know what I'm saying here. Thanks for reading.