It sounds to me like you solved your own problems.
you said he was reduced to a Blank Slate
that means his memories were either removed or erased (possibly with a copy). Then you said he went through some sort of rehabilitation creating a new personality (depending on your time line this could be weakish or strongish, childlike or adult, etc.) Which means, at least to me, that your character when reintroduced to memories of his past life will react as anybody would to memories that have somehow entered their head by drinking a potion or having a plug inserted into their skull.
The memories feel real enough (maybe) but the continuity between who that person was and who he or she is now would be broken so it is likely, by my estimation, that they would treat such memories as alien to them-self.
What the results are is irrelevant at this time, does the character become a new character or a reawakened old one bla bla)
It would be a trickier thing to pull if his memories were suppressed in some way especially if they manifested slowly but the end result would be a new character. Philosophically the logic behind this is that the character can never be the same after his memories change the same way that people could change their body/mind/whatever to look younger but can't actually be younger without traveling through time.
From the perspective of your character assuming he was not told the whole truth he experienced his childhood (a very different childhood from normal but childhood none the less) and was introduced to new memories or told about his previous existence from a book or person.
On the other hand your the writer so go whatever direction you want with the guy is he a side character have a nasty laugh at his expense or maybe just a placeholder for the previous character (depending on your audience's point of view a older character or new character). do the new memories override "his" memories? they could if you say they can especially if you justify it in some way.
EDIT: see also Heel-Face Brainwashing
and Brainwashing for the Greater Good
edited 22nd Oct '10 4:16:34 PM by GiantSpaceChinchilla