What is the name of this book series?

[Mark H]

I have read this book about a year and a half ago. It was about a young magician that summoned a very powerful demon and was scared of him at first, and the demon wanted to take advantage of him. The boy was adopted by a very powerful man in, if I'm correct, central Britain. Each demon has their own special power and can change shapes and take on forms that can slip under the human eye. These magicians can see onto the plane that these demons exist on. The man in which the boy was adopted helped the boy develop his demon summoning and controlling powers every day or week, if I can remember the plot correctly.. Can any one help me figure out what the name of this book is? I know it is part of a series and, if I am correct, this book is the first one in the series