Okay, I'd say that orphan was probably Sentinel Prime - he might have been ignorant of his destiny at first, then later found out about it - just like Optimus. After all, if he grew up with no other Primes around, then it's possible.
Reasons to support this:
+ Sentinel Prime can transform, unlike the original 7 Primes.
+ The original 7 Primes all died, save for the one who became the Fallen - who eventually died from shame (loss of face

).
It's possible that Jetfire never knew the name of this orphan and so when Sam mentioned "Optimus Prime" he may have assumed that it was that orphan whereas in fact he is the orphan's descendant.