9 year old son with bad habit?

It is not a habit, he needs to do it. It is probably not his fault because if you smack him, he wouldn't want to be smacked again. You should take him to the doctor to check his respiratory system. You should be gentle to your kid, and autism has nothing to do with his grades because if he wants, he can pay attention and learn, so he might have that problem(I hope not!) If you take him tot the doctor, and he still does that, get ketchup and say it is hot sauce. It will not hurt the kid because it is not hot sauce. Put a little bit on your finger every time he does that and say you will put it on his mouth, BUT DO NOT USE ACTUAL HOT SAUCE! Hope I help!