* "Leave of absence" is a noun phrase. It should be used as a direct object, not a verb.
* "Files" needs a direct object. It needs something to be filed.
Here are some ways to correct the sentence:
* "When he officially files for a leave of absence." This uses "files for" as the verb phrase and "leave of absence" as the direct object.
* "When he officially goes on leave of absence." This uses "goes on" as the verb phrase.
* "When he officially takes a leave of absence." This uses "takes" as the verb and "leave of absence" as the direct object.
The most accurate and grammatically correct option depends on the context and intended meaning.