The NSW Board of Studies lists the following ways in which pencil grip is accepted
• having the thumb and index finger pinch while the writing implement rests on the middle finger;
• having the thumb and middle finger pinch with the index finger resting on top of the writing implement;
• having thumb, index and middle fingers share the task equally

I wouldn't say that you are teaching your child incorrectly, just make sure that his writing as he further develops it is legible (nothing worse than trying to read children's writing sometimes) and try and make sure you use foundation writing so that he doesn't become too confused at school