I don't know. I have a hard time believing Harry would take the risk of going to a police consultant for advice on how to let his budding serial killer son get away with murdering people.
He wasn't murdering people back then, so it is reasonable to believe that Harry might have pursued other avenues first, one of them being to ask one of his friend's (Matthews) friend for help.
That might be how she got hold of young Dexter's drawings.
"Look, my son is drawing lot's of this creepy stuff. Got any advice what it might mean and what I can do about it?".
Exactly. He's got a troubled, traumatized, little kid. He feels he's in over his head, so he discreetly consults an expert he trusts. He probably hadn't even come up with the idea of the Code at that point. He may have still been searching around desperately for some way to "treat" Dexter--only to have Vogel explain to him that psychopaths couldn't be cured?