have you noticed if him & the jill get on, does she snap at him i ask because i have a jill who is very food motivated & although theres plenty put in for them she hides it & wont let the other ferret have any , she squeels at the other ferret & so after a few days of observing this i seperated her , the other ferret on inspetion was geting thin.
Just wastch them & see how they behave , only thing i would do at this stage is to seperate them & see if you cant tempt the skinny one by hand feeeding, maybe have him indoors , if he isnt getting any food or very little