First of all, love that you watch K-Drama! I love watching them, but I haven't had time to watch most of the episodes of Heirs thus far. But the fact that Lee Min Ho stars it made me think twice since people tend to overestimate the quality of a drama/TV show/movie if it has a famous and popular actor in it. He's a great actor and his looks aren't bad, but I just don't understand why people think he's so great. Personally, when it comes to looks, I think Lee Seung Gee, Kim Bum, Choung Kyung Ho, Jung Yong Hwa, Taecyeon, and Kim Hyun Joong are better looking than Lee Min Ho. Of course they're not as famous, but I believe that they are hotter.
Anyway, sorry about that. Now back to your question. The song that you are referring to is called Here For You by Big Baby Driver (빅베이비드라이버).