He didn't forget, it's not amnesia, he didn't lose his memories. He repressed them. The memories are still there, given by the fact the he is slowly getting them back. It can happen during tramautic experiences. He was very young 6-8 years old and he saw months of abuse of his mother by his father, his sister injured in an accident that partially his fault, then taken away from his mother and sister. that is a lot to take in for a small child. I sure as hell buy that he repressed all that shit, I know I would.
P.S. :I for sure would like to repress Ryker and anything that has to do with him.