he is young.
At 2.5 years old they are not even fully developed socially.
At 2 years old, they do what is called "parallel play."
Get some books, and read about Toddler development per age.
When it comes to "sharing" expectations.... it needs to be congruent with a child's age stage.
I, have never FORCED the sharing issue. With my kids. I keep my kids' age in mind and their developmental stages.
AND, I ALSO taught my kids, per age stage, that they do not have to.... share, nor everything. Meaning, they also have to learn, that some things are not to be shared. If they have something special to them, they can put it aside. And not have to, share it with a kid that is at our home for a play-date, for example.
I never FORCE my kids, to share something. And I also taught them, HOW to discern, what they want to share or can, or should not. And I also taught them how to say it. ie: "That is special to me, I don't want anyone to touch it..." or "Please don't touch that, it is special to me...." or "Mommy, I don't want anyone to touch that...."
My kids didn't learn not to share, but rather, HOW to share. And per their own cues, too. And their own boundaries. Too.
My kids share. Many things between themselves as siblings and with friends. But they also know, they can opt not to. If something is special to them or "valuable" to them in their own child way. Fine.
They KNOW they can tell, me.
It is learning boundaries.
And certainly, if someone grabs something from them, with or without asking first, they don't have to, let it happen. But if the kid is younger than them, my kids fully understand that a toddler will not "know" "how" to share, perfectly. They know, situations, of sharing.
That is important for a child, to learn.
Your child is so young. To be an expert at it.
Its okay.
He won't be like that forever.
He is at that age stage.
And they can be territorial at that age.
It is a phase.
But you teach the child, about social boundaries, how to communicate it, how to say it, and how to also know their own cues or tolerances for things.
I as an adult, well I have a special coffee mug that I like to use. I don't want anyone else to use it. Fine. I don't have to share it. I tell my family. Its no big deal.
Things are to share, or not.
And everyone will learn in time, that not everything, has to be shared.
I wouldn't want anyone else to use my "special" things.
But in toddlers, they may pitch a fit about it.
They are not articulate yet. About socializing or communicating. Or boundaries.
It is taught.