My son keeps standing inside the little goal to stop shots

I would step in, but I would not tell him he is doing something wrong. He has solved the problem in front of him: stand in the goal and shots are easier to stop. The issue is that he is now getting far fewer chances to chase, tackle, dribble and attack. I’d give him a simple challenge such as, “Can you come out and win the ball before it gets to the goal?” That still values his desire to defend. You can also use training games where there is no advantage to camping on the line because the goal is wider, there are two goals, or the direction changes. The point of no goalkeeper is that all three children get a fuller game. I would gently help him experience that rather than wait months for him to discover it by accident.