This story I think is set in California, the boy or young man is living with his ailing mother, there is an old house with a back gate, he goes through this gate and enters another world.
This 16-17 year old man/boy is now in this new world and is taken in by witches who think he is a half breed and is there, in this world to be trained with others like his kind. He is given a pile of boards to build his hut. The boards all must fit together to form the hut perfectly with none left over to prove his first test, he builds his hut with one board left over, all the other trainees have none left over.
Later in the book, he goes on journey to Xanadu and the crystal place described in Samuel Taylor Coleridge’s poem.
Later, he comes back to our world, and he never gets wet because he is able to walk between the rain drops.
At one point, near the end of the book I think, he goes looking for his imprisoned girlfriend. She is imprisoned in a bottle of wine, but she is imprisoned at the 13th hint of flavor. He drinks the wine and travels down through the worlds via the flavor’s and must stop at the flavor his girlfriend is at, find and saver her and escape.
I think it’s by Clive Barker or Dean Koontz.