Could it be Darren Shan's Lord Loss, first book of his Demonata series?
The books start as one day, Grubbs Grady is being called out of class for smoking and then he is punished by his mother for it at home. He finds out his sister Gret told on him and gets her back by putting rat guts on her towel. For this he is grounded without any video entertainment. Then suddenly the situation changes, he is no longer grounded and he is sent to stay with his aunt after Gret apologizes to him for telling and being given several presents. At his aunt he sneaks out at night and goes home to find his family slaughtered by demons commanded by Demon Master Lord Loss, using magic he escapes and ends up in psychiatric care where nobody believes him about the demons. Then one day his uncle Dervish shows up and believes him and helps him get out of the institution. He goes to life with him and meets Bill-E, who thinks that Dervish is his father and a werewolf. They try to find out how he controls himself but as they do, it turns out Bill-E is the werewolf and Dervish, his friend Meera and Grubbs barely survive the encounter due to his foolishness. Dervish explains everything and they decide to do battle with Lord Loss, the only one who can cure Bill-E. One will play chess on five different boards at the same time against Lord Loss while the other fights two of his familiars, if one of the two loses all three will be killed. The two player and the one they intended to cure, which is what happened to Grubs parents and sister. Dervish plays Chess while Grubbs fights the demons, then as he is about to lose Dervish abandons the game and saves him. They convince Lord Loss to allow them to switch places and now Dervish has to fight the demons and Grubbs who isn’t all that great at chess must play Lord Loss, after losing two games. He learns that the way to defeat Lord Loss is to deny him sustenance for Lord Loss feeds on grieve, fear and anxiety. He starts playing disinterestedly and recklessly and becomes increasingly indifferent and using this strategy he beats Lord Loss who cures Bill-E. However there is one final part to the deal, one of them has to go into Lord Loss’s realm and do battle with him, while their body stays behinds as a zombie. Grubbs takes care of his soulless uncle for a long time until one day he returns and plays a prank on him, by making him belief he turned into a werewolf himself. Luckily for now it was only a prank, and his uncle returns as normal.
From the article on the Demonata:
Dark Calling reveals that both the Old Creatures and the Demonata are creatures of the original universe which preceded the two universes in the series. This universe was structured like a chessboard; it consisted of sixty-four squares, half of them White and half of them black, each square containing infinite space within. The Demonata inhabited the white squares, the Old Creatures the black; the different zones were kept separate by a force which the Old Creatures called the Kah-Gash. While the Demonata were capable of reproduction, the Old Creatures were sterile, but this did not bother them; since time did not exist in the original universe, everything therein was ageless and immortal. They can still get killed.
Title found through a search for site:goodreads.com boy family "killed by demons"
