I highly doubt he is an experiment of Orochimaru. When did Orochimaru get the Sharingan? And if he did, why didn't he implant it in himself like he always wanted? Doesn't make sense.
The chances of him being Itachi's son are much higher though. And I'll probably side with this option for now, but still not very sure about it.
And just another Uchiha survivor could be possible, but in that case, he should be infuriated with Itachi for destroying the clan, not Sasuke.
And being an outsider with an implanted Sharingan also doesn't seem viable since he wouldn't be concerned about the Uchiha clan and bitter towards Sasuke.