Your once over-possessive toddler has now turned into a ready-to-share preschooler. Still, he is in the process of coming to terms with letting go of his favorite toys or books for a short duration. Sharing might depend on the person, object and mood of your child. It is therefore important to encourage your little one to take turns while playing and give every kid a fair chance.
What you can do
To inculcate the habit of sharing, the child’s need for reciprocation should be respected. Let your son go to a friend’s house with the host’s permission. If he feels the friend did not share, pacify him by saying that the friend is still learning and an understanding child like yours must demonstrate sharing to them.
You are still his best teacher hence lead by example for him. Whenever you borrow something from your little old, use words like “May I?” and “Thank you”. Similarly, when you share your things, tell your little one “I am happy to share my things with you as I know you will use them responsibly”. If your child does not want to share some of his belongings, never force him. He will learn in due course of time.