Translation:According to him, this dress is black.
OK, so all of them are either Genitive or Accusative. But no, they're not the same.
The basic, most neutral form is 'go'. "Lubię go", "Potrzebuję go".
Then you have the more emphatic "jego", usually showing some contrast. "Lubię jego, a nie ciebie!" "Potrzebuję jego, a nie was!".
And then there's "niego", an example of what I call "n-forms". The n-forms, if such a variant exists for the given pronoun, are used only (and they're obligatory then) after a preposition. "Idę do niego". "Liczę na niego".
And of course "jego" means "his" in all cases, but that's a different thing.