No he doesn't know it yet. Sang zhi hasn't told him yet.
SPOILER!!!
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It happens when zhizhi finally tells her parents about her relationship with xia ju and they very strongly oppose to it. Mainly because of their age gap and xia ju is an orphan..so zhizhi has a huge fight with them and returns to yihe, with those little paper stars in her hands..and shows it to xia ju. Basically confessing the one she liked when he was 13, her older online bf...was actually him all along. Xia ju remembers zhizhi's crying face on the airport that day...so it was him that made her cry like that. He felt bad but also overwhelmed because sang zhi has loved him for so long...ig they also do the did after that. Heh ε=ε=(ノ≧∇≦)ノ
So when's he gonna know that she was in love with him all along and the 'boyfriend' was actually him