I don’t think you’re doing anything wrong except expecting him to respond to “No” too early and too quickly. It doesn’t just happen with one or two repetitions. Right now, walking and doing things on his own are very important to him—chances are, he is in motion a good bit of the time. Those are good signs of developmental progress that you don’t want to squelch. At your son’s age, words need to be strengthened with action. Every time you say “No,” try to get down on his level and put your arms on his and keep them at his side briefly. Repeat your “No” and name the forbidden object: “No, you may not have mommy’s billfold (or whatever).” Then be sure to give those busy little hands something else to hold and manipulate. In time, the word “no” will come to stand for the entire sequence. Finally, make absolutely certain that he doesn’t have a hearing loss. That will need to be checked repeatedly as he gets older.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.