Liam discounted his song on iTunes this week so it surpassed Louis right now in the US (why couldn't you wait a week Lima!) Niall as well though his has been discounted for a while!
Just keep buying, streaming, and telling your friends and family members to buy and stream. Louis should get a boost when the song fully impacts radio on Tuesday and when he does some promo in the U.S., which will hopefully be very very very soon. In addition to the discounts, Liam and Niall have had the advantage of a lot of U.S. promo already and a lot of radio airplay.