If you spawn him at night, he will spawn, but there will be no event triggered. Killing him does not spawn a chest, does not give event reward chest nor the mastery point.
If you spawn him at daytime, when Itzel Outpost is still infested with Mordrem, he will not spawn, but the event will still be triggered. You can see the HP bar in the event box top-right, but he is nowhere to be found.
If you spawn him at daytime when Itzel Outpost has been cleared, he will spawn normally, and an event will be triggered. Killing him now will grant you the chest, event reward chest and mastery point.