Why did he say "You are right about him" to Batman?

That was the Flash.

One of Flash's powers is he can run so fast he can travel through time. I think that scene was not a dream sequence, but Flash warning Bruce Wayne of a possible future.

I think that scene hints at a comic book called Injustice Gods Among Us. It is an else world where Superman becomes a dictator after the Joker tricks him in killing Lois and their unborn child.

The Flash said something like Lois is the key and you were always right about him. I think the him he was talking about was Superman. I think that Flash came from a world where Superman loses Lois and becomes the dictator like in Injustice Gods Among Us. Which is what Batman always feared of Superman.

EDITED as of 9-21-2021

Looks like my suggestion all these years has been confirmed by Zach Snyder

Snyder said the sequence is a byproduct of the Flash (Ezra Miller) using the cosmic treadmill and creating a rift in time. There was going to be scene in a later film in which we would have seen Bruce Wayne and Cyborg deliberating on what point to send the Flash back to so that he can prevent Lois’ death. Snyder revealed that it is Lois Lane’s death that leaves Superman open to being infected by Darkseid’s Anti-Life Equation, and the incident that would have led to the Knightmare future the Justice League would have attempted to avoid.

reference https://collider.com/batman-v-superman-knightmare-scene-explained-zack-snyder/

EDITED as of 11-30-2021

Confirmation Injustice God's Amongst Us was Snyder's source of inspiration for Batman V Superman's nightmare scene.

reference https://screenrant.com/batman-v-superman-knightmare-injustice/

The scene is when Batman was dreaming when a fuzzy Flash comes and says:

Bruce! Listen to me right now!
It’s Lois! Lois Lane! She’s the key!
Am I too soon!? I'm too soon!
You were right about him! You were always right about him!
Fear him! Fear him and find us. You have to come find us, Bruce!

When I saw Batman v Superman: Dawn of Justice after I saw Justice League, I thought this scene was about reviving Superman.

Batman was going to revive Superman, but Aquaman & Wonder Woman disagree.

Here, Flash is saying ... :

... Lois is the key (not Martha, not Kryptonite), because that is how Superman controls himself.
... Is it too soon ? Yes, because Superman is not yet buried, which Flash assumes probably because of clues in the room or because of the reaction of Batman.
... Batman is right (it will revive Superman), always right (only Superman can fight that threat) hence he should not get dissuaded by Wonder Woman.
... Fear him, because Batman should not assume a safe revival. Fear him because Superman will be in a murderous rage when revived.
... Come find us, which ties in with the scene with Wonder Woman, where Batman talks about finding other Super Heroes.

At this moment, Batman may assume that he is right about Superman being a threat, and hence prepares to attack.
Much later, when Batman, Aquaman, Wonder Woman & Flash talk about reviving Superman, Batman knows that the message was about that moment.

When Batman tries to revive Superman, he uses this message and brings Lois, else he may have brought Martha or even some Kryptonite weapon.

How did Flash know this ? Because he was there and had seen it all.

Why did Flash send this message ? It is like the Terminator situation, where John Conner sends back Kyle, because he knows what happened in the Past. It may be elaborated in some later DCEU movie.