i think since the first time they met, hes already interest in her, the curiousity abt some feeling he never had before makes him more and more falls for her, maybe around chapt 10 and in chapt 11, he realize all the noises he hears all this time, (you know he could hears peoples breath, walk, etc, thats why he killed people) could be gone when he holds her, so yeaah after that hes really straighforward to her abt his feelings