J.E.
Don't worry. He sounds like my oldest did when he was his age. The first born doesn't learn to share as easily because he doesn't have other kids in the house to share with. Just keep exposing him to other kids and other times to interact with them. Eventually, he will want something they have and you can explain again why sharing is important. What I have found that helps is to provide praise any time they share ANYTHING. You can say, "Thank you for sharing the pillow with me. Now we can both lay here while I read you a story." You can also reverse it and when he wants something of yours you can say, "Of course I will share my crayon with you. We can both use the red crayon. Isn't it fun to share." I know it sounds cheesy, but the more you positively reinforce sharing the easier it will be for him.