Unfortunate, but c'est la vie. How do you prove to NW that he didn't know the first account was opened? How does NW know that the first account wasn't opened with his knowledge and then you decided to cash in on the bonus by opening another account?
I'm not saying that's what happened, but put yourself in NW shoes.

And please don't use the "take care of the new customer" whine. Customers should be treated with value whether they are new or old. Where do you draw the line with that? "I'm a new customer and I thought the $500 non-refundable airfare was $50, you should refund me the $450". If it was your business would you give them the $450?

Again, I'm not saying that this is your attitude but it seems your last comment is heading that way. Now, if NW had done something wrong, then absolutely I would expect them to correct it. In this case they did nothing wrong. And you can't say that opening the account in April was wrong as his sister-in-law did exactly what you did in May by opening an account for him. Granted you say it was without his knowledge but that is shame on her for not telling him. It is in no way NW's fault.

Lastly, they are absolutely correct that they cannot discuss it with you. It is not your account. If you have power of attorney, then send it to them and I'm sure they'll gladly discuss it with you. Otherwise, like I said, unfortunate but c'est la vie.