Internally something is wrong. Gas or pressure is being released and the eye is the weak point in a fishes body. Not much you can do to help the situation other than provide him with pristine water conditions and feed him well.
I had my female punk my little male and he swelled up when she rejected him-he got traded in for a new male and he eventually got better at the fish store-the other male has been coupled with her for the last 3 yrs-long story short-he'll get better over time