If the amount of time between your explanation and his was somewhat significant it could be that he forgot you saying it and stored it in his brain, and then when it came up again he thought it was his.
Santtu's idea was specific and meaningful to sound propagation.
It's not like he gave a common idea, such as how to broil a steak.
Certain types of ideas don't just get "mistaken" for your own.
I say the guy knew damn well what he was doing in swiping his idea.
Those who can't think for themselves, steal.
I've seen it too many times, in business.
If this is the best of possible worlds, what then are the others?
― Voltaire, Candide
For what it's worth, it seems to be a feature of INTJ/ENTJ relationships that the ENTJ is in the more vulnerable position. There's some technical stuff to explain it, but either way, it's something I've noticed, to my chagrin. The INTJ's going to have a little better control over the expression of his feelings but not really enough for him to be confident about dealing with what an ENTJ wants to express. Both get stuck. (And the other side of the coin is the INTJ is totally screwed when it comes to putting things into action together with the ENTJ.)
I'd still mention the origin of the idea to him. The discussion won't go well unless you're both really aware that you'll both be defensive about owning your own work, and maybe not even then.
ENTJ: "I liked having had that idea, it was cool."
INTJ: "I worked on it a lot before I published, man!"
ENTJ: "Sure you did, I know, I just liked that I knew enough to create something new too."
INTJ: "It's my fucking paper, man! But jeez, if you want I'll mention you next time, whatever."
ENTJ: "Do what you want, it's none of my business. You're really a prick sometimes, you know?"
Yeah, it happened to me too with a professor. What can I say? I don't even think it was one of my best ideas, so I didn't care. But if you think it was a good, original idea you should confront him. Be prepared to nothing happening, though, since there's no way of proving it was your idea, unfortunately. The worst part was that the prof. said that he didn't like the idea, it didn't really seem sound or realistic, etc. then here it goes, he publishes a paper on it, LOLZ.
I think that academia teaches you to be ruthless with this type of thing (that's also why I'm never going to get into it, I'd prefer to cut wood). I remeber reading an article on Scientific American where a post-doc said that she doesn't write on her blog what she's recently found out because the only time she did, a colleague stole her idea in 2 days.
Other anedoctal experiences: I think that professors are paranoid about imitation and copypaste from papers. I've been accused 3 times, so far, of having "copied" something (without the professor checking out on the internet - just at first sight of my paper) because the idea I had was "too good" (????????). It seems that many of them are resigned that every student should be a mindless non-interested drone, and they are all-knowing gods.
At least you could use the fact that your idea was worth writing up after all to pressure him into acknowledging that A. you were right that it was interesting and he was wrong to initially dismiss it, and B. you obviously have an effing awesome creative intelligence since he luuuuuurves your ideas so much. This goal would sidestep his likely insecurity in the more-obvious conflict over who deserves the credit, while satisfying your former desire for his approval.
I talked this briefly with my gf today and I'm in peace with it now. Probably an honest mistake, plus if the idea was too "unproven" for him the first time, it probably just entered his subconscious, until he had forgotten.
I was trying to help him with this thing originally, so I was never seeking more than acknowledgment.
My other INTJ did a same kind of thing with an innocent idea I thought - when I told I had said it so many years earlier, he laughed and said, yes I had.
Lots of good ideas are tossed around- talk is cheap. Formalizing them and doing the work is the hard part. Fundamentally its not your paper (vague ideas do not count as coauthorship) but it may be appropriate to mention your name in the acknowledgments section. Nonetheless, if it really is your idea, the INTJ should at least confirm this verbally to you as a friend, even though in the grand scheme of writing a paper that's not nearly enough to make it "your work". In my experience, those that cite generously are usually the better researchers- they have enough ideas to go around.
Who said it was vague? I did enough work to make it obvious it was workable and did the required thing better and with fewer resources. Formal proof lacked, but it was enough to be worked on with knowledge it'll be a big improvement. Besides, like the amount of work done is saving anything. Most of the great ideas can be condensed in few good paragraphs. Given that, it's easy in a way to write up the rest - little lateral thinking will be required, though it will require a great deal of other kinds of thinking.
Actually, I wonder why he hasn't been able to introduce much of an improvement over anything, apart from the obvious stuff. What he's accomplished is one slightly novel method. He mostly summarizes people's findings of a subject, gives a cross-section about the current research to a topic - and does quite basic work, with the minimal innovative step and quite maximal amount of writing per unit of innovation expressed.
What could I say, he's like an accountant in scientific process. Some papers he showed, I was nice to him but wondered if was preparing for a textbook or doing research. As if he was afraid to do a thing of his own. Perhaps not afraid, perhaps just unable.
His favorite thing must be the proof by exhaustion, his goal the finding of all the dead ends.
I could say, I take things like that for granted. Summary of prior work has it's uses.