At this point, we do not know if Dr. Rolf was involved in this at all, so serum is not even an option. People DO suffer from amnesia after traumatic accidents...sometimes getting their memories back, sometimes not. HAS Jack been in a coma all these years? Or living somewhere with NO name, or being known as John Doe? Until we get more info from Eve, who is not parting with any, it seems, we can guess all we want.
Currently, Jack is overwhelmed, trying to absorb what these people piled on him. Father, grandfather, brother, husband....even grandchildren.......a lot to take in. And it does seem whatever Eve told him doesn't completely jibe with what others are saying.
And thank goodness he already had a small memory. Hopefully more will come, tho nothing is said in spoilers.