When he brings you something that he has created, the best thing to do is to describe what you see, in a warm, appreciative voice. Listen eagerly if he begins to talk about it also. When he is done, you don't need to critique the drawing. I love it. Perhaps the most crucial situation in which you need to tell your son that you are proud of him is when he feels like a failure.
Children often feel like failures.
The most important time to tell your son that you are proud of him is when you see him deflated over his lack of accomplishment. Describe positive character traits that you see. And I'm proud of you for trying. You could have kept on playing, but as soon as you realized you needed to go, you ran to the potty. You make me so proud and happy.