No doubt he was convinced he was saved, but on your theory he was wrong. You seem to make a big deal out of this assurance of salvation, yet it's no assurance at all because the only way you can be sure is to die without having sinned seriously. If you are logically rigorous, you'll find yourself perilously close to the Catholic position. I mean that the most you could have would be a moral assurance of salvation, not an absolute assurance. At any one time, you could say to yourself, 'If I died now, I'd go to heaven,' but you could never be absolutely sure that you'd always be in that condition.
Catholics say the necessary condition is the state of grace.
So long as we're in the state of grace, we have a moral assurance of reaching heaven. But we know we can sin seriously and lose grace and throw away our salvation. There's only one consistent line for you to take. You can't say the born-again Christian has an absolute assurance of salvation and, at the same time, say that we can't tell who's saved until death intrudes. That would mean we'd have to keep the assurance in suspense until death, which means it's no assurance at all.