The poet says, “I would not intrude on him” because he does not want to intervene in the natural process of learning. He wants the boy to learn the meaning of loss on his own. He also doesn’t offer him money to buy another ball because that would be worthless. He wants the boy to learn the lesson of responsibility.
Yes, the boy has had the ball for a long time. When it bounced into the water, all his memories of the days of childhood flashed in front of him. This led to a realisation that those moments would not come back, just like the ball. He can buy new balls and can similarly create new moments, but those that are gone would not return.
“In the world of possessions” means that the world is full of materialistic things. Here everything and every action is made to possess something, whether it is the possession of land, property, money, or any other thing. The poet suggests that losing a ball, which is a very small thing, would make the boy understand what it is like to lose something that one possessed
No, it seems that the boy had not lost anything earlier. The words that suggest so are ‘He senses first responsibility in a world of possessions.
The poet says that the boy is learning to cope up with the loss of the ball. He is experiencing grief and learning to grow up in this world of possessions. He learns that there are so many things in life that are lost and cannot be brought back. He is sensing his first responsibility as he has lost the ball. The boy will learn how to stand up and leave the losses behind as he would have understood the true meaning and nature of loss.