Please answer this form to the best of your ability.
How big is the tank?
Is it uncycled or cycled?
How much water do you change?
How often do you change the water?
Please try to answer these to the best of your ability. If you do not have a test kit, please bring a sample of tankwater to the nearest local pet store. They should test it for cheap, but most of the time free. Write the actual numbers down, don’t take ‘okay’, ‘low’ or ‘fine’ for an answer.
I can't see the attachments unfortunately so no help there.
If the white spot on his belly (between his head and his anal fin)? It is not unsusual for bettas to have lighter bellies which would explain why it expands when he eats.
Does the white spot have a fuzzy, cottony surface?
Its hard to tell with his gills unless I see them but it doesn't sound like any disease I've heard of.
I will be able to tell you a lot more when you fill out the consult form.