The only bonus I would personally allow would be allowing the invocation of his lycanthropic aspect on spells relating to that nature (e.g. those involving animals, tracking, nature and violence) and I'd allow that regardless of the time of the month. During the 5 day period I would actually propose compelling that aspect for spell failure (The character is supposed to become a prototypical Norse Berserker - prone to bouts of uncontrollable raging fury and not well known for composure or control), or at least to compel the character to put excess energy into fallout when the choice wouldn't be optimal.