The problem with my son, who is 5 years old, is that he does not concentrate on his work. I mean he is pretty restless. and this problem has also been noticed by his school teacher who has pointed this out to me yesterday.

Please tell me how do I make my son concentrate on his work.

There's a book called "Creating Kids Who Can Concentrate". Not sure of the author, but it may give you some ideas.
Sit with him, give him rewards (breaks, snacks, fun activities) for completing a set amount of work, rather than a timed amount of work.
Make sure everything else is right. Kids will only concentrate when all other needs are met - look up Maslow's hierachy of needs.

Is he having trouble concerntrating doing homework? It's often best to do it straight after school so he's still in 'school mode'. Play will help him concerntrate better too, children often spend hours on one game.