First off it is fungus which is not good.
If it were me I might prepare a mix of 3/4 water and 1/4 H2O2, Drugstore/ pharmacy 3% Hydrogen peroxide and use a q tip to accurately, carefully dap a moist not dripping q-tip swab on the spot, in an away from the eye motion. You obviously wouldn't want to get it in his eye. While holding him with a tank water moistened folded paper towel. Or cloth without fabric softener chemicals. They can absorb through their skin.
If that method if too much to try then try Pimafix. If you can buy a small pack of Indian Almond leaves / IAL there are some ones packaged listed for Bettas which are small size. But if someone you know has them, or a independent LFS you might be able to buy a leaf and use a chunk of it. It has antibacterial properties and could help the water.
If the eye is missing I would treat try API Pima fix
API® Pimafix Fish Fungal Infection Treatment
https://apifishcare.com/product/pimafix
There are stronger options.
The most first things would be to do a couple of water changes to get the water as perfect as possible. Any time there is a death or illness that would be #1 to make sure.
Also make sure you do not over feed and leave food chunks on the bottom of the tank to degrade and muck it up.
You can remove bits of food in a smallish tank with a turkey baster. Or by vacuuming the substrate with a vac wand for a bigger tank.
