Thing is, if the exchange of force involved really was anchored to his body somehow, then it's absolutely true that he wouldn't need to be wearing metal to fly, just something to repel off of. But that's the same statement as that the fact that he needs to be wearing metal to fly is evidence that said force exchange is not anchored to his body.
So, yeah, when he stops a slew of bullets, the kinetic energy of the bullets doesn't throw him back. If there was any evidence that he could be, say, dragged around by someone moving an object he's levitating, then you'd need to invent some opposing force he's using to stabilize himself when that doesn't happen. But without anything to suggest that, there's no reason to make it the default assumption, and when it's outright debunked by the flying thing, it really, really seems arbitrary and non sequitur to me.
I'm hazarding getting sidetracked, since Whizbang's original point in mentioning conservation of momentum was more about how our intuitions work in sussing what a superpower is doing on screen or on page than something more like a "scientific explanation" of the thing. But I just kinda feel like it's pretty natural to assume that the momentum isn't actually being set off anywhere and maybe emdrives work in this universe, or that the thing being manipulated is actually the Earth's own magnetic field as Whizbang suggested, etc., as opposed to the force passing at any stage through Magneto's actual body. I think that's generally my default assumption with telekinesis in fiction unless something indicates otherwise.
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.
she / her / her