R.W.
He doesn't understand. It's great that he has a way to ask for what he wants. It's okay for him to experience disappointment, and to cry. Parents usually want to prevent unhappiness in their children, but it isn't necessary---they learn about feelings, and about how life works.
I know a young child who was taught to say things like "Bye bye playground, see you next time!" to help him understand that he could leave a place where he was happy without thinking that it meant that he could never be happy again.
I'm thinking your son could think that way--bye bye toy, time for a different toy.
At his age, ownership has no meaning, sharing has no meaning, so no need to get too sophisticated with your explanations to him.