Go to the fish store and ask for an antibiotic for finrot/popeye. Mardel Trisulfa or BettaFix might work. Follow the instructions for water changes and dosing. Might add a bubbler to give him more oxygen too. Those things can happen from bad water quality so you should get a test kit to make sure water perimeters are okay.

I know i'll get some flack for this but i would recommend keeping him in at least a 5 gal tank with a sponge filter. The little fish bowls are prone to problems. And follow evesta's advice he needs immediate help.