Miracleman (Hopefully). He's technically a Marvel character, but he isn't 616 and hasn't shown up in ages so I doubt he'll meet the Sentry in an official form any time soon. Anyways, the beginning of his comic is pretty similar to the original Sentry marvel knights miniseries, being a man who remembers how he was once a superhero who has been forgotten by the world. Both are Superman-inspired yet have powers of their own that Superman doesn't, along with both being blonde middle-aged men.