Sharing

We are having a real issue with sharing lately. My son is super possessive and while we are all possessive of our belongings and rightfully so, he needs to learn that he has to share his toys with his little friends when they come over to play. I try to explain to him when he goes to “so and so’s” house that they let him play with their toys so he must do the same when they come over to play at our house. Instead of constantly saying “no” to him, I have tried saying “it’s ok – you have another one here” and passing him another toy. If it is a favorite toy of his, I will ask him to find a different toy for his friend to play with and then he can have his special toy back. I tell him that we are going to take turns. That first “so and so” will play with it for a little while and then Jax can have it. I have told him to be gentle (because he tends to grab his toys a bit aggressively as of late) or “it’s not nice not to share” although I don’t think he understands the concept of mean and nice just yet. Other than what I am doing, I don’t know what else to do. I hate having to constantly interject and feel stressed out at a play date. Any suggestions on how to get through this stage a little easier?