Sharing is a complicated concept for toddlers to grasp and can take several years to master, so be patient. But be sure to praise any behavior that even remotely resembles sharing — holding up an object to show a playmate even qualifies. Make sure he sees you praising other children who are showing signs of sharing too.
Letting your own actions show him what sharing is all about can be the best teaching tool of all, though: When you’re playing with a game or eating something delicious, ask him if he wants you to share it with him. Over time, he'll imitate you. For now, try not to get involved when toddlers are possessive with each other's toys. They'll usually work it out themselves.