I think what Julio's grandfather means when he tells him ''keep the river in your heart and follow it'',he means to follow what his heart says. He should do what ever he feels like doing. Julio's Grandfather tells him that maybe because he wants Julio to be happy, and not have regreats later on in life.
I think that Julio is taking his grandpas advise, because he does what he wants. Julio will fallow his heart because it looks like he is really into Romiette. He will go far with his grandfathers advice.