Personally I don't think that kind of Magus is necessarily out of place, I do think it needs some limitations (and perhaps a better description of its mrchanics). One thing I would suggest (and you've touched a bit on) is that it could consume more energy for him to use other's Magus, that way, at least if he somehow got into battle with another character he would have a bigger disadvantage. 

As far as the science behind it: he could replicate the psychic energies of a target, thus essentially copying their abilities (though this would most logically be done through physical contact,  which again couldmean: more contact = quicker absorption of energies = less time required to mimic abilities). 

Just some thoughts,  will review this post when I get in touch with my computer.