Lets see.
INTP - thinks about words and meaning a lot (too much), but also predisposed to social awkwardness with weak Fe and high introversion. When I've considered saying I love you to people, it's been either one of 2 situations: one, I feel extremely connected to the person; two, I feel very intense, but it's a mix of attraction and anxiety. The words become a tool to help a person conquer (maybe attenuate) their own anxiety and insecurity. It's not always the case the the second is mixed with anxiety, but it's true for me, and I don't really know anything else. (boo fucking hoo, Edahn)
One is feeling LOVE, one is being IN LOVE. They're different, even though we use the same words. One is a feeling of closeness that you'd have with a best friend that you love, another is something more akin to lust.
The way to tell the difference, I'd think, would be to examine other cues of cognition and emotion aside from the meaning of his words: body language, word choice, rhythm of speech, and quality of humor/creativity. You have to be perceptive/empathic to pick up on the cues, though. Does he look like he's trying hard, or not?
(BTW, since this thread is about INTPs only, we can keep it here for now, but if it broadens beyond type, I/someone'll move it.)