Disclaimer: I own nothing but my own OCs, should I create any for these stories.
The trouble with regeneration was that it basically reset everything. Favourite foods, tastes in clothing, thought processes, mannerisms - nothing was safe.
It was, quite literally, the death of an old personality and the birth of a new.
Not even memories were sacred - often times, these would get pushed aside in favour of establishing a new personality in the form of a blank slate.
So when he first woke after that horrible ordeal that was his forced regeneration, he couldn't remember anything.
He knew that he was missing information; knowledge that he should have retained, but for the life of him he couldn't recall what it was.
In any case, though, this was a new life - a new beginning, as it were. He'd just have to muddle things out as he went along, and hope that he remembered things in time.