You can only retriggering him by advanceing the windows time... the game somehow keeps track of the year you are at... if you return to normal time later the Boss probably won't spawn in October once more... that aside the special accessoire he drops is a one time drop so you can't give your entire party anti poison protection... or atleast that was the caser the last time I checked... same applies to any special enemy
I started playing when Winterhowl special came out in December, used the in game cheat book, not the modded one.
Played it so much that I came to year 2 and did the Halloween event twice, by normal game time expanding.
That boss spawned once each year when pumpkins was all around and my party has 2 of his rings.
Restarted with this new version and am still in year 1 so can't confirm if it's the same for this version, but I would expect it to be.
On another note:
If you have Bianca with you as a permanent Team Member, you can use her PickPocket skill to steal from foes and get a double drop of what they have, meaning if she steals it before they die, they can still drop the same thing when they die, also from most bosses..
It works against The Profet, so I got 2 Tainted Dawnbreakers, upgraded because of getting 2xdust with same Bianca tactic..

However it didn't work against the Halloween Boss because his ring is found on his finger AFTER the fight.
Am trying to compile a list of where to get resources and how to increase the gain by using Bianca's PickPocket atm, gonna take some time due to the massive amount of content
