He has already read "Very Short Introduction" by Peter Higgins.
Many thanks in advance.
Andrews has a book titled just "Number Theory" which is very combinatorial (gives a combinatorial proof of Fermat's little theorem) - the second half of the book is about partitions and their generating functions.
Borevich and Shafarevich have a book also titled just "Number Theory" which is an introductory text on algebraic number theory, ideal class groups, etc., and by the end it uses these techniques to explain how Kummer proved Fermat's Last Theorem for exponents below 100.
Baker's "A concise introduction to the Theory of Numbers" is exactly what it says on the tin. It has five to ten exercises per chapter, and each chapter is a fairly short introduction to a different branch of number theory, giving examples of the fundamental techniques or ideas or results in that area.