Make Your Kid And Their Friends Play The Sharing Game
One of the vital lessons your child should be learning as he grows up at this point is how to share with others. Your kid will be naturally possessive of his things, and it is your job to show him that sharing is not only good, but also fun.
Make your kid play the sharing game with their friends. Give every child a toy and make each child ask another kid politely to share the toy they have. The other child should then willingly give their toy to the child who asks for it, and then ask someone else for their toy. Once somebody gives their toy, everyone should clap and cheer for that kid.
If your kid refuses to share his toy or play the game, do not force him. Let him sit out for some time and watch the other kids playing. Once he sees how much fun they are having, he will want to join them himself. Once he does so, make sure you praise him.