He is gorgeous! I can see why you don’t want to lose him. What size is your tank? Get a test kit for ammonia that uses drops and test daily, do large enough water changes to keep ammonia at 0, and you should be fine. It will be labour intensive until it’s cycled unfortunately. It may be good to proactively add 1 tbsp of aquarium salt per 5 gallons, might ward off fin rot which can set in really fast on a betta in an uncycled tank. Also, dose Prime daily for the whole tank volume to detoxify any ammonia and nitrites.