The Wormworld saga. Volume 1, The journey begins / Daniel Lieske.

Enhanced descriptions from Syndetics:

This gorgeous fantasy epic follows Jonas, a young boy from our human world, who stumbles into an alternate universe through a painting in his grandmother's attic. When the portal closes behind him, Jonas must find another way home and begins a journey through this strange and fascinating land. Along the way he meets Raya, who becomes his guardian in the new world. But there are many things Raya is not telling Jonas, and this world is not peaceful.

Raya takes Jonas to the bucolic waterfall village of Ankal Aasha and brings him before a priest who tells him that his destiny is to heal the rift in their world. Raya and Jonas travel to the city of King's Peak, home of the Worm Mountain, under which, according to legend, the great fire god Unurtha is buried. Here they find that the city's people have turned a blind eye to the dangers of Unurtha and are using a crack in the mountain to harvest energy from the fire and create new kinds of energy crystals. It is Jonas's job to stop this. But in a world of mounting violence, monstrous creatures, and shifting allegiances, what can one small, scared, human boy do?