There is of course the unfortunate fact that for some people living here, the conditions aren't that different from the Developing World anyway, and he's pretty arrogant to forget that he may have patients who are dealing with the same issues per housing, income, food, etc.
The other responses are all good. If I were you, I'd follow up with him. He's still "just a resident," and if you give him some cited research and let him know that you are concerned that he have accurate information to share with his patients when he has his own practice or etc. .... Maybe that will make a difference.
Or you can ask him *what research has he ever found which shows harm done* in bf beyond a year of age, and maybe he should do some more research.

: Because he won't find anything showing harm, and will in fact find plenty about benefits.
(Since when is breast cancer only an issue in the Developing World, for instance? IIRC the most benefit occurs when bf beyond a year? And that is a definite OB issue).