Poor diets can and will cause fur discoloration(the food dyes in these cheapy foods will cause discoloration of the fur and stool, not to mention making both the ferret and it's poop stink); it's much more noticeable in white ferrets and those with light colored coats. Also, some albinos and DEWs yellow as they get older; just a part of aging. If he is that filthy, he may not have had his bedding changed very often. Bathe him(you may have to do it more than once) with a decent ferret shampoo. Also be sure to scrub his tail well as that dirt buildup and blackheads can make the fur fall out causing "rat tail". If the tail dirt/blackheads don't come off immediately, try scrubbing the tail with an antibacterial soap or an acne facial wash(Clean n' Clear works). You can also add a drop or two of Dawn dishwashing liquid to your ferret's shampoo to help get the oil out.
If after bathing him, he continues to feel and look oily(and smell stronger than usual), that may be an early adrenal disease symptom.