Uhm, this isn't quite what you asked for -- John is temporarily absent, but teen:Dean has to take charge -

Brother Mine by etrix

http://etrixiswriting.livejournal.com/52273.html
http://etrixiswriting.livejournal.com/
Brother Mine [master post]
http://archiveofourown.org/works/520206/chapters/919418

Summary: John was out on a hunt when the call came: Kate Milligan was dead, and John was named in her will as the guardian of her son—his son—Adam. Pissed but knowing his duty, Dean pulled Sam out school during exam week, and they went to Windom in the thick of the Minnesota winter. 7-yr-old Adam was a lot like Sammy at that age, but Dean wasn't ready to accept him as another Winchester. Then the boy's life was threatened and all bets were off.