It is difficult to tell from the photo but he may have fin rot. Does the edges of his tail look black and ragged/melted? If not he may be getting fin nipped. Have you noticed any fish picking on him? The last thing it may be is a genetic deformity and if that's the case then I wouldn't breed him.
King PK Wasabi; HMPK Frostini; 5 CT Bettas Jewel, Shimmer, Opal, Scarlette (aka Ms. Piggy), Pixie; 1 SD Neptune; 1 HM Gypsy; 1 DTHM Pebbles; 4 Orange Von Rio Flame Tetras, 5 Bleeding Heart Tetras, 4 Harlequin Rasboras, 2 False Julii Corydoras Salt & Pepper, 1 Bristlenose Plecostomus Hoover, 2 Pictus Catfish, 1 Albino Rainbow Shark, 5 Dension Barbs