I know poor AA does not normally leave his house and therefore you can't adopt him. But I have been reading through the Unofficial Skyrim Patch notes and came across this NPC/Creature Fix in v1.2.3 from October:

"Aventus Aretino lacked AI packages to send him back to Honorhall Orphanage like he was supposed to be. (Bugzilla #5054)"

So, for those of you that have this or subsequent USKP patches running plus HF, I'm really curious: If you do see Aventus at the orphanage now, does he give you any adoption-type dialogue when he's there?

EDIT: Found answer = NO in an October discussion thread:

PrinceShroob said, "Oh, and FYI everyone: Aventus being in the orphanage does not make him adoptable with Hearthfire. He has a unique voice. "