He definitely could have been nipped (only you would know this) but whether he was nipped or not, he now has fin rot. How do your water parameters look? Do you have a qt tank? You can try treating with healthy water, aquarium salt & melafix and see if this has any effect. The problem with fin rot is allows for opportunistic infections to set in due to the damaged tissues. Melafix & salt have some antiseptic properties but if infection has already set in, they will have little effect and you will need to treat with antibiotics to prevent it from spreading into the base of the tail. Damaged tissue will regrow over few mths time (if its not being nipped) but if the rays (bones) break off, they will not regrow. I cant tell from the pic but if there are broken rays, i would skip the melafix & proceed straight to salt & antibiotics in a nip-free environment. Keep in mind that antibiotics will have an effect on your bb.