Hi
you don't need to focus on the word and put your efforts on erasing the word from his mind, instead it's better to help him find his potentials and strengths by providing the appropriate conditions. focus on his likes and dislikes. for example if you notice that he's into arts and painting try to provide him with the environment to develope his creative ideas; in this way  his mind unconsciously will avoid the idea that he is an idiot. i mean the idea of being an idiot will be backgrounded, on the other hand the cleverness will be forgrounded in his subconscious mind.
the direct method cannot be that helpful; i.e to implore him not to say this.
of course the source of the problem can be varied from the type of friends he has and the language they use in their community to his place and acceptance among friends or classmates and etc.

add a line to his statement, such as: and a talented writer.
then ask him to write you a story about and Idiot named Toidi.
then the next time he calls himself an idiot add a different statement to his comment,:and a good listener. Keep adding good characteristics to his and he will soon see himself in a different light.  good luck. MelInda

I would first start out by asking your son what he believes the definition of an idiot is.  Then ask him what in his behavior leads him to believe this.  Let him know that we all make mistakes and point out some you have made.  Then tell him mistakes do not determine who the person is , only how they respond to them.  Them tell him that he came from you( God) depending on your spiritual believe and that God does not create idiots and for him to erase the word from his vocabulary.  Tell him to identify the error that causes him to feel this way and discuss ways to correct it.  Hope this helps.