My guess is that Conyer's sharing of his govt vehicle with family members is nothing too uncommon among Congress folks (or federal and state officials in general). Both parties, no partisanship here. Breaking the rules, but they treat it like a "perk".
But I also suspect it is total malarkey that Conyers didn't know the kid was driving the car. The response from Conyer's office just reads like playbook template verbiage to cover his butt.