When you explain, you teach him how to move the decimal point. When you move one position to the right, you tell him that it has increased tenfold, and so on. When you move one position to the left, you tell him whether the number is getting bigger or smaller, and then tell him that it is ten times smaller than the original number. Just talk to him, don't start work, so that students may not understand. After you start teaching him, he will find a pattern.