Nothing wrong with banjo whatsoever!
My first assumption would be the fishy smell is from his anal glands. Relaxed, they are being...um...emptied. Soemtimes the stink can then attach to the dog and the room for even a day. It is gross, but it's normal.
Now unless Banjo is itching and scratching in his nether reagions, that itching and scratching has nothing to do with this smell. Same as if he has a skin infection of any sort, the smell is entirely different.
All you can do is wash his rear end when it happens if it is needed (and only then) and get some air freshener or Nilodor or something for the house.
Re his itchy skin etc...how often ARE you washinghim? And with what?