The way you have phrased the question, I'd almost have to say "Not until he found the shirts." It's the application of THAT WORD which makes me put so late a date on it. WAS he in love with Jack? I would say as soon as the feeling of friendship had been translated into passion....by the seoond night in tent.
Maggie (B73) has an interesting theory. She thinks he realizes Jack was in love with HIM when Cassie says "Girls don't fall in love with fun." In other words, he realizes that just because he's "no fun," (sometimes)...would no more keep Jack from loving him than it did Cassie.