It's clear that Jumin is most likely touch starved (I mean, we know what his mother would do to him and I don't think his father is very big on affections) so I believe that Jihyun being the affectionate friend he is, is a miracle for young Jumin. Maybe the former is perceptive enough to figure it out but even if he isn't, Jumin is incredibly thankful for the habit Jihyun adapts of throwing his arm around the other boys shoulders, amongst other things.