Well there are no organs that far back in a betta so it isnt some sort of organ failure...you may be dealing with a tumor which is a bit of a problem. No real treatments exist for them you see.
Tumors will be solid flesh all the way through and can be either acute or benign. benign is ungrowing/unchanging and acute is still spreading. if the tumor settles down then it is possible your boy can continue even with it for some time as long as it doesnt affect his quality of life. If it is spreading and it begins to affect his organs or his bouyancy however you will have problems with his health.
As long as he is alert, happy and being looked after by such a lovely person I am sure he will be able to go on without concern, but be prepared that there isnt really anything that can be done to remove it, fish surgery isnt very advanced.
Of course if at some stage you realise it IS full of water or air you are in luck, because thats a cyst. Fish can deal with them as long as they dont bother them but if it bursts on its own (or you feel its safe to do so yourself) it will purge the gross and go back to normal. I am rooting for the second option.
