I am trying to remember the title of a book about a boy who learns magic.

It was so long ago all I can remember is:

• A young boy is being taught magic.
• He goes on an adventure.
• He gets stuck in a painting.
• In the painting is a powerful book of magic which he begins to study.
• He finds out how to get out of the painting.
• On his way back to his village, he somehow is travelling in a flying castle and pays back a man that he borrowed or stole a boat from during his journey.