When did your LO learn how to share? Any tips on how to help him understand?

DS is 17 months and there is a boy at daycare just over 2. He and DS go at each other almost every day. They fight over toys, usually smacking or biting one another. They take turns provoking the other. The other boy seems like he might be a tad behind developmentally (just a feeling I get, really) but he is more verbal and listens to simple directions. DS just doesn't get the sharing thing. I talk to him about it and taking turns, but he just doesn't understand quite yet.