there is quite alot of possible answers. he may die if no more grow because of the lack of oxygen getting to him, or he may have to have special requirements ie: super airated water, he may also gulp air alot more. more may grow as he is only young but generally they are born 'fully gilled'. are these your personal babies you hatched? or did you buy him? pics would help. oh as an after thought maybe if he was kept with other babies they might have nipped his gills off.