The first mistake you are making is by punishing your child for not sharing. This is a common misgiving that lots of parents’ have- that if they force their child to share, the child will readily accept it. Never force a child physically or by using threats to share his toys.
Try and talk to your son as to why he doesn’t like sharing his things with the younger ones. One reason could be that earlier on, your son might have been forced to share his things with the little one forcibly. Many parents tend to give in to the younger child when they start to throw a tantrum or start crying. Another reason could be that the younger child always wants her brother’s special things like maybe his art book from school, or his favorite car. Even between siblings, it is all right to have special toys that one doesn’t like to share with the other.
Another facet is that the older child may not be interested in anything that the younger one has, because he has outgrown those things. Hence he does not feel that it is required for him to share anything with the younger sibling either. In this case, tell your older child that he can share things he has outgrown because he really doesn’t use them any longer. This way he will resent the sharing less.
You must also pay heed to the relation that the babies are sharing which could be the reason for the frequent sharing problems. Make sure that no one child feels more special or superior over the other. Lastly, be patient. Sharing takes time to learn.