Depends on what memories were missing and how did he come up with the conclusion that some of his memories were erased.
Personally, I wouldnt make the MC freak out much if he realized that his memories were missing, because he doesn't know about what memories were lost BUT he will be in distressed undoubtedly. Instead, it's when his memories recover will he freak out.
For example, MC lost the memories of his lover; he knew that he's missing someone but doesn't know who exactly and that puts him in a precarious state of mind, without even knowing that the person he's missing was the person he coldly brushed off countless of times. However, when his memory recovered, he obviously remembered his lover who was always beside him the all time but he kept pushing her away, so obviously he would be distressed and try to find her but unfortunately, it's too late... for she has moved on. Something like that I guess.