As for Dex telling Harrison, I'm starting to wonder if it would be a good idea at this point. Harrison and Dexter have major issues to work out before he can begin confiding in him. Harrison is an angry teen, who's emotions change with the wind. 95% of his anger is directed towards Dex, there's no telling how he'd respond right now. Especially if gets high again, he'll tell the whole damn town
What Dex needs to do is break down his wall, and allow Harrision to confide in him about what he's experiencing. Maybe he could even talk about his brother Brian and how the death of their mother impacted him, turning him into the ice truck killer. Still keeping his killings out of it, but allowing Harrision to feel comfortable enough to share what he's going through. He can use his brother's situation to mold Harrision the way Harry did for him and teach him the code, and how to control his rage and how to direct it. Dex needs to be a father, he doesn't need another kill buddy. We've seen how those turned out, lol. Harrison needs direction because he's heading down the wrong path. Keep in mind, we still don't know what really happened while he was in Argentina, or in foster care.
By the way, does he not remember Aster and Cody? Why weren't they an option for him to stay with in Miami instead of foster care?