"He will go to Japan in two weeks."
This is the grammatical rule when specifying time. 后 and 前 are the common characters that are supposed to be placed after the specified time.
Playing around with the given English sentence, we have:
- The subject is placed first in the sentence.
- "In two weeks" being placed between subject-and-action, and then worded as "two weeks after".
- The action "will go to Japan" is placed last.
So the worded sentence is literally "He, two weeks after, will go to Japan". Translate, so we have the answer.