The simple rule is that FIDE rates games where at least one move has been made and the FIDE Laws of Chess applied. You can read the full rules here. That means that his next FIDE rating will be based on the games he has played so far this month plus any more he might play. The only way you can gain or lose rating points is by playing FIDE rated chess.
Does that also mean that Nepo's and Aronian's results will be converted as win(without rating changes) because otherwise it will be unfair in a round-robin situation?
This is covered in the FIDE Tournament Rules:
6.6 Round robins
(1) Each player has entered into a contract to play throughout the tournament.
(2) When a player withdraws or is expelled from a tournament, the effect shall be as follows:
- If a player has completed less than 50 % of his games, the results shall remain in the tournament table (for rating and historical purposes), but they shall not be counted in the final standings. The unplayed games of the player are indicated by (-) in the tournament table and those of his opponents by (+). If neither player is present this will be indicated by two (-).
- If a player has completed at least 50 % of his games, the results shall remain in the tournament table and shall be counted in the final standings. The unplayed games of the player are shown as above.
So, completed games will be rated. If Carlsen has completed 50% of his games then his remaining opponents will have their results against him recorded as wins for the tournament but not rated, since no moves were played. In effect he has defaulted his remaining games.