Drew McWeeny of Hitfix is reporting that:
HitFix can confirm that he will be playing the part, and that Zack Snyder has already finalized his designs for the character so shooting can take place soon.
Hitfix also thinks they have a line on how much screen time Aquaman will have in the movie as well as his reason for being there. They're saying that it will be a smaller role, similar to a cameo which will be expanded upon in the Justice League movie.
But why is the King Of Atlantis showing up? Global Warming, that's why. Or something. Hitfix again:
I'm not sure if that's the actual name they'll use or not, but what we're hearing is that he is not pleased about the World Engine and what it did to the Indian Ocean.
Khal Drogo as a pissed off member of Green Peace? I'm down with that. I like to think he'll role into frame to the Whale Wars theme before eating the heart of some Eco-Terrorist working for Lex Luthor. Boss.