I mean, i guess the writers couldn't do much with him. For all we know, shredder probably gave him up or he ran away [most likely because even someone like the shredder horrified him]. Or perhaps, they could have mutated him or something like that.
Or considering you get mutated into what ever animal that you touch, because the DNA is still on you. Xever mutated into Fishface because he touched that fish he wanted and the slime from it was probably still on his hand. Bradford turned into Dogpound because Hachiko bit him and the dog's saliva was probably still on his hand.