While the Inquisitor does get to say "I love you" to Dorian as part of the final Trespasser crossroads
dialogue, I don't believe Dorian ever responds with an "I love you, too," nor is there a clear "I love you" from him anywhere in the actual in-game dialogue. As Gervaise says, the closest we get is, "You are the man I love," during the scene where you discuss his returning to Tevinter.
There are a few things that work in favor of an audio splicer in this instance. Ramon Tikaram's (Dorian's VA) natural voice is what he used for Dorian. He has also done several audio books, in addition to films. There could certainly be audio out there from some other media of him saying, "I love you, too" that the person used.
There are also a lot of Dorian fans out there, including those that are pretty familiar with the mod tools and audio extraction. If that clip was in the game, I'd think we might have heard it before now.
That said... I suppose there always is the possibility that it IS in the depths of the game files somewhere. There are a ton of audio files and it would be tedious to go through them ALL to find one single nugget like this. However, I'm pretty sure it is not present as a response in the actual game.