Misappropriation of Ideas does count as plagiarism
Plagiarism does include using others ideas without appropriate credit, even though copyright (or patent) protection does not cover ideas. (This is one of the ways in which the legal notion of copyright [or patent] violation differs from the ethical notion of plagiarism. Another is that copyright and patent deal with ownership, whereas plagiarism deals with failure to give fair credit.)
Looking at the provisions of the NSPE code that you cite, it seems to me that the most clearly applicable provisions are III 9 and 9a, which reads:
9. Engineers shall give credit for engineering work to those to whom credit is due, and will recognize the proprietary interests of others.
a. Engineers shall, whenever possible, name the person or persons who may be individually responsible for designs, inventions, writings, or other accomplishments.
Since you are working with the NSPE Code, I would suggest that your state chapter of the NSPE (which will have a name of the form, "Alabama Society of Professional Engineers") would be one place to try. State chapters vary in their activities, however. If you and the engineer in your story are both NSPE members, I believe you could lodge a complaint about him with the NSPE.