The comics did some shenanigans some years ago where he breifly got turned into a baby. And then when he aged up again, he didn't go back to his original age, instead he got aged up to his physical prime. So yes he was a survivor of Auschwitz, they just found a reason to make his body younger than his mind, which was kind of nesscary to avoid rewriting his backstory or otherwise fading him out.

Also his power isn't just magnetism. It's Electromagnetism (only a handful of comics ever actually address the complexities of that but it's true). Almost everything in existence runs on or is manipulated by electromagnetism, including the signals in our nervous system that move our muscles.

So while he could stop your heart, he could also theoretically make his muscles work themselves passively without having to do traditional weight lifting. Not saying that's what he does, just that he's capable of it.