RE: Deck to be competitive with Dark Hammer theme deck
The best advice I can give would be to buy your own theme deck to play with him. All of the current format-legal decks are pretty evenly matched against each other, barring weaknesses. That said, don't pick a deck like Resilent Life, since the Psychic types will hit Fighting for weakness and the Fairies will resist his Dark types. If your nephew plays a lot , especially if he starts going to a League, he's going to want to build a stronger deck before long though. You might want to talk to his parents (or do this yourself if you're his guardian) about buying better cards and building something semi-competitive, or at the very least a deck with some Pokemon-EX (even if they're not good ones) and viable Trainer cards so he doesn't feel like a sitting duck. Kids especially can get discouraged when they show up to a League with just a theme deck and can't win a single game.