I read this book around 2010 and I need help identifying it.
The first scene was of a woman who had just given birth (?, she had her newborn) and it was raining. A wizard (who have been called something else) knocked on her door and she let him in. He was there to give a gift to the child. He was already in a bad mood because of the rain and the mud. The mother was trying to get her son something good and she annoyed the wizard who ‘gifted’ the kid with truthfulness before leaving.
In the next scene (?) the boy is older and having difficulty with his inability to lie. There might be some sort of fair at his town. He talks with his mother and somehow it’s decided he’s going to magic school.
At the magic school (which I think was underground) he is working with another boy, who he is on vaguely antagonistic terms with. They either have to extinguish a flame or make it cold. The kid who can’t lie casts a spell (?) and when he picks up the candle it is cold. The other boy sits on the table and starts screaming because the heat was left behind. He accuses the first boy of doing it on purpose, getting him in trouble (?) from their teacher.
At some point both boys and the original wizard end up in tunnels on a flying carpet being chased by giant worms. (The worms might be purple, but I’m not sure.) I think the original wizard wasn’t very powerful.
I think the cover was yellow and it may have been part of a series.