I just PM-ed you a bunch of article links, as I'm not allowed to comment them directly. I recommend you print out a stack of articles in this vein (they're all peer-reviewed, published in good journals and very easy to find), and whack them on his desk. He's also definitely misunderstood the articles he's quoted to you, as explained by other people below (I'm a psych graduate too and can vouch for that).

Honestly though... I don't think this is about these one or two papers he's going on about. I don't think I've seen anyone else address this either, so here goes:

Have you considered that this might be an insecurity of his? You say he is Korean and came to the US at the age of 9: perhaps he had trouble in school or got bullied for speaking another language? It's actually quite likely that he got teased for not speaking English right off the bat. Kids are cruel, and he may just be really scared your child will be treated in a similar manner.

It may be worth gently reminding him that your child will be learning English from birth, living in an English-speaking country from birth, and will definitely be totally fluent and have absolutely no accent. I can totally vouch for it- hell, I know little kids, with french parents who only speak french to them and send them to french schools, who speak English with a British accent just cos they live in the UK!

Also, the world has moved on quite a bit over the past 20 years, and bilingualism is definitely an asset in this day and age. This has been detailed at length by people all over this thread, myself included. In fact, I'd argue in most metropolitan cities with immigrants, most people speak two languages.

I feel like he's being a bit unreasonable, and if he's not usually like this about things... I'm willing to bet it goes deeper than "I read this one article that claims this one thing and I refuse to budge on it".