N.S.
Here is what I have been doing: talking about sharing and taking turns ( and modeling that behavior) , also making sure that my son isn't always getting first pick (at dinner, of the blocks, whatever), and letting him wait for some things (so he gets the idea of delayed gratification)
I role play with his stuffed animals too.
I let him know that I notice when he shares and plays nice with other kids, just in everyday situations.
It has all been working.
Maybe let your son know that he doesn't have to share certain things (like a lovey) so he feels he has some control over his life.
I think the reading the Mister Rogers book about making friends helped too.