You're describing Kernel Fleck, one of the main protagonists of the Demonata Series by Darren Shan. The plot you describe is that of the second book in the series, Demon Thief:
Kernel Fleck is a lonely child. He has always been different: he can
see strange patches of light in the air around him. Other kids think
he's mad. But when he manages to link some of the lights together, he
creates a window into another universe, and disappears for several
days. When he returns, his memory a blank, his panicked parents whisk
Kernel and his younger brother, Art, away from their home, off to an
isolated village, to start a new life.
One year later, Kernel is enjoying himself. Life is good. Until one
day a crazy old woman summons a demon. The beast slaughters many of
the children, then kidnaps one of them and retreats back to its own
universe. The child it has kidnapped is Art.
Can Kernel find the courage to step through the window after the demon
in a desperate attempt to rescue his brother? And if so, what wonders
and terrors will he discover in the barbaric universe of the evil