So, he knows he is Jack Deveraux, but doesn't remember his life in Salem???
Back some years, Jack ran into Steve, an orderly at the hospital where Jack was a patient. So, he told Steve who he was really, brought him back to Salem, eventually Steve remembered. That was Stefano's doing.
Now, AM GUESSING, Eve sees Jack, tells his who he is, makes up something about being her husband, boyfriend, lover, whatever, and brings him back to Salem. Yep, NikkiFan, recycled, a variation on the theme.
Also really hope this is not the case, since Melissa Reeves (Jennifer) has said when she saw the script, she was shocked, never has seen anything like it.