The book was likely aimed at middle-schoolers, and I read the first half of it in the early 90's.
The boy was a successful thief, and I think he came from an abusive home. An old wizard/magician found the boy and convinced him to follow the magician on a quest. I think the magician sought the boy out because the boy had potential to save the world. The magician started to train the boy in the art of magic, and promised him that he would be a great magician some day. The boy got captured and... that's about where I stopped reading.
I think this book was the start of a series.
Edit for future searches:
- Yes, this book was part of a series, a trilogy to be exact.
- The boy was a thief, and he did come from an abusive home, although without blood relatives.
- An old magician found the boy by happenstance, and lured him into his house. The magician made the boy his apprentice, then invited him on a quest a couple of months later.
- The magician didn't seek the boy out, but he did recognize the boy's potential to be a great apprentice. And it wasn't really to save the world, just to do a real important mission.
- The boy did get captured, along with the magician, on their quest.