Since he is new...some of the problem may be related to the adjustment period and then with the distilled water....those chemistry changes without proper acclimation can sometimes really stress them to the point of no return and not a lot we can do once its done......But we never give up too soon......
Give him some time, quiet dim lit location for now, maintain water quality and temp, still water, warm, humid air over the water, offer live food to stimulate the feeding/hunting instinct and to help support the immune response....patience......
Be sure and remove any uneaten processed food-but if he doesn't eat the mosquito larva right away-leave them-the larva movement should stimulate his instincts to hunt and eat...he may just need some time....
If you have a brine shrimp net-use that to collect the larva-that way you can get some really small ones-but they can eat much bigger larva than you think.....rinse them under running water then place them in a small cup of dechlorinated water that you can cover-use a eyedropper to feed to the Betta-if you place the container in the fridge they last a bit longer-but check before you open it so you don't let mosquito out in the house...lol.....
If you have any IAL or naturally dried and fallen from the tree Oak leaf...add this to his water for the tannins...