One could make the case that maybe the magnetic fields don’t actually originate from him, but that his powers consist of an ability to manipulate, re-direct, and strengthen the existing magnetic fields in the environment. So he’s intensifying the Earth’s fields to hold up the disk, and there’s a third-law force-pair between the disk and the Earth. He could, in principle, pull the frog trick on himself, but maybe he’s not powerful enough to produce a field that strong. But this is pure fanwankery; Otto of course has the real reason for the disk (it just looks cool). The question is just then whether it was the writers’ aesthetic judgement that led to it, or Magneto’s.