Betta Fish: This is Ruger He is very feisty he flares up at my friend all the time, and he only likes flakes. He does not like pellets he was $30 but he was worth it

Have you tried blood worms or some live foods?
The reason I ask is I recently lost my female after two years to swim bladder disease. I read that flakes can expand in their stomachs and cause problems. I don’t know if that is true or not, but I felt bad afterwards. I fed her a variety of foods, but I had given her flakes shortly before she declined.