I tried. You can't. =(

WHO ELSE BUT BETHESDA!?

I can't adopt him. I can't get him to initiate any conversation after I've helped him.

Actually I did have this exact same thought earlier. Yes he's still in Windhelm, but funny thing, he does say "I'll return to the orphanage once they've had time to...clean up the mess." So......

Yeah I wanted to adopt him but alas ti's impossible.

Vampire family.

I am perplexed that Aventus is unadoptable, but I can give four possible reasons.

1. Programming oversight. He was supposed to be adoptable, but Bethesda simply forgot to add in the necessary code to make him so. This could be corrected by a routine patch.

2. Glitch. Like the Falkreath land that apparently no one can buy, Aventus's adoption dialogue is bugged and will not come up. This could also easily be patched.

3. Voice acting oversight: Whoever was doing the voice acting forgot to record the "Aventus Adoption" conversation. This is more problematic, since the child actor who played Aventus might not be available.

4. Deliberate decision. Bethesda actively decided to have Aventus remain unadoptable, for totally unfathomable reasons. He obviously needs a good home, and a lot of people expressed interest in adopting him.

Assessment: If 1 and 2 are the case, I expect that a patch will soon be made available, maybe fixing the Falkreath issue and letting you Aventus fans take your little darling home. If 3 or 4 is the case, well, there's plenty of other younglings to choose from.

yep. kinda creepy and cool at the same time

We talking vanilla vampires or Dawnguard vampires?
Cause if it's vanilla vampires a family of Werewolves would be way more hardcoe.

Can't you find him a new home or am I confusing a different quest/games?!

Aventus says he will eventually move back to the orphanage, but nothing has happened yet, there are no options to adopt him. Braggen really wants to adopt him so I will wait and see what happens.

A fellow Necromancer! Agreed, I wish I could train my dear, adopted son Francois Beaufort (unofficially a Delacroix though) in the ways of Necromancy...or at least magic in general. Hopefully the option to train our adopted sons and daughters in our paths will become an option in the future.

if i could he would join the ranks of the DB just like me

It is rather odd... first, he never returns to Honorhall, even though two of his dialogs say he wants to. And now, no way to adopt him.

Maybe "Aventus Aretono" is ancient Cyrodiilic for "glitch magnet".