Yeah, that's grade school advice.
Either:
(1) confront the coworker on this and tell him to stop or be cut off of information from you. Not always possible to do this. Also creates more immediate tension in the office.
or, my preference:
(2) use email to show the information trail,
with your boss cc'd every time. Where it's acceptable to your company, also cc the cute civil engineer (contractor) on your calculations. Make sure to let your dick co-worker know, and the contractor civil engineer know, that dick may feel free to present
your calculations and if they have any questions, you'll be glad to answer them. The boss will soon have the real picture of who's doing the work and has the technical capabilities, and who's the pretender, all without noticable drama. Same for the cute civil engineer contractor.
