Your questions is clear. It is not uncommon for writers to paint themselves into a corner. I once sat on a story for 2 years before figuring out how to end it in a way that would not crush my protagonist, because he didn't deserve that. It took a while, but one day it just came to me how to end his story, not his spirit.
Alice Sebold made telling a story from the afterlife popular in "The Lovely Bones" in 2002. It's an overdone motif now.
But, you could reincarnate your guy and not tell your audience until the end that he is speaking through his reborn self that emerged as a baby born at the moment of his death and is able, somehow, to "remember" his prior life. Or perhaps his spirit jumps from his just-dead body to the nearest fresh one like Brad Pitt does when he personifies Death in "Meet Joe Black" using the body of a guy killed in a traffic accident who, like Jesus (according to believers), enables a super-being to experience what it is like to be a living mortal.
Some authors might inform readers at the end that they have been reading a dead character's journal all along (a little like S.E.Hinton does in "The Outsiders"), although you'd be adding the footnotes, I guess. Maybe his wife is writing the final chapter after he's gone. Or maybe you could reveal at the very end that the protagonist is "acting" in 1st person to tell you about the person whose identity he stole...he would have had to have known the deceased very well (best friend or relative) and anyone who does this is already ethically challenged and knows how to prevaricate, so he could just be telling the reader directly - or through an interrogator - about the person whose Social Security number and the life attached to it he stole. Maybe your protagonist is an impersonator whose widow-now-wife was in on it all along and your readers don't know until the end that, out of guilt, your protagonist is using first person to tell the story of the guy he is now pretending to be, and leave the paranormal stuff out.
Please consider looking into the TV series "Legends" with Sean Bean and "Forever" with Ioan Gruffudd in leading roles, Both deal in their own way with being something other than they appear.
For every one of these ideas, there must be 10 more that have already worked or could.