I agree with all the ladies when they say that it's not going to be cheaper than it is here. However, since LV has the same pricing policy worldwide (if it is in the same currency), the duty free boutiques will charge the same price as they do here. This means that if you purchase your goods in US $, you may be able to get a better deal because it's tax free. (the prices can seem to go up and down if you purchase with the Korean won, because duty free shops always base their pricing on US $ and then convert that according to the daily currency exchange rate if you wanted to pay it in Korean won)
If your brother is going to go check things out for you, I'd recommend the duty free shops they have in the city (is he going to Seoul???) not the ones at the airport. They tend to have a pretty large inventory. If you want, I can give you a list of places he could go. PM me if you need the info.