In "New York Serenade" Hook helps Emma regain her memories of who she is so that she can help her family escape or break the curse that has been placed on them back in Storybrooke. While nobody is sure how or why it happened, they still can't remember anything. Prince Charming claims that they had just said goodbye to Emma and Henry, and the next day Snow is almost nine months pregnant judging by her huge baby bump.
While Everyone seems to remember who they are, just not how they all got back to Storybrooke, there is still one person that doesn't; Henry. Emma doesn't want Henry to regain his memories because he would lose all those happy moments that he thinks he's had over the past year. She doesn't want him to remember all the hurt and anguish that he's gone through since being adopted, then finding his mother, and learning all about his fairytale life. It's confusing, abnormal, and has caused a lot of pain.
Morrison says that as much as she tries to fight it though, there comes a time when it's necessary for Henry to retain his memories so that he can help save the town. Henry is kind of like the story keeper. He holds the book full of everyone's truths and how they affect everyone and cross paths so it's imperative that Henry has his memory so that he can help others that might be coming back to Storybrooke remember who they are.
Apparently, we are going to see some quality time between Henry and Hook and without knowing who he is that could prove difficult.