Problem is malnutrition, possibly overfeeding and chronic poor water quality.
In a tank/bowl of this size three water changes a week are needed.. 50%, 50% and 100%. He should be scooped in a little plastic solo type cup and not use a net. New water should be the exact same temp as old water - matched using his in tank thermometer. Water and conditioner should also be premixed before using (you can use fresh water jugs from the grocery store - rinsed well in hot water but not chems).
However, 2 gallons is really minimum to keep betta's healthy long term, and betta's kept in small bowls of this size live an average of 2 years, compared to double that+ in larger containers.
What heater are you using? You need one that can keep a constant unchanging temp between 78-80F and you need to be using an in tank thermometer to monitor the temp
He should be getting two small meals a day, at least a couple at a time and one fast day a week. Blood worms are like feeding potato chips - very high in fat and very poor in nutrition. They should be fed no more than one once a week in replace of a pellet meal. Also, if yours are freeze dried they need to be soaked thoroughly in a cup with some tank/treated water for at least 10minutes before feeding. You should look for a pellet whose first two or three ingredients are whole fish and not fish meal or wheat. How many you feed depends on the pellet you choose.