Teething can cause foul odor, and full adult dentation won't be in until sometime between six and seven month so this is a possibility. Eating feces, some puppies will, whether their own or another animal's, can also give the same results.
Finally, don't overlook an anal sac origin--dogs will groom
this area and the odor has often been described as fishy/dead. Obviously, such gasto-intestinal problems as parasitism can also cause foul odors, but I'm assuming that you've had this puppy appropriately dewormed and vaccinated.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124