Oh, Nonny. You had to go there. After I showed such remarkable restraint on my last ask, you had to ask the question guaranteed to make me ramble on for a century with all my thoughts, feelings, and theories. Very well. You asked for it. :D
Now, this is a really interesting question to think about and actually somewhat challenging to answer, because Brian genuinely didn’t believe in love. He couldn’t even conceptualize it. So while he may have been feeling all these emotions toward Justin that people generally associate with falling in love, Brian himself wouldn’t have recognized it as love even if it bit him on the ass (which, come to think of it, I’m sure Justin did).
My short answer is this: I think the first time we witnessed a true spark of love from Brian was during 1 x 07, and it built steadily all throughout the first season, and by the end of 1 x 17 he was absolutely in love. Now, that’s not to say he always accepted those feelings, or acted in a way that somebody generally would when they’re in love, because, come on, he’s Brian Kinney. As for when he actually realized it? I’d place it at prom, and one specific instance at prom, but I’m going to put that below the cut because believe me, I will ramble. However, we all know what happens at prom, and following those events, Brian tried to drown out the idea of being in love from his mind, mainly with drugs, liquor, and plenty of sex. But by the end of 2 x 02, I believe we’ve reached the point where Brian can’t deny to himself that he’s in love with Justin, even if he won’t use those exact words.
So, that’s the short answer. If you want the long answer, complete with pictures and a nearly episode by episode description of Brian falling in love with the incredible Justin Taylor, let’s hop along under the cut.