You may have heard the Toddler's Creed by by T. Berry Brazelton:
If I want it, it's mine.
If I give it to you and change my mind later, it's mine.
If I can take it away from you, it's mine.
If I had it a little while ago, it's mine.
If it's mine, it will never belong to anybody else, no matter what.
If we are building something together, all the pieces are mine.
If it looks just like mine, it is mine.
There are many variations and additions that have been added over the years.
He's getting that he's a separate person with separate wants and needs, and a sense that he may actually have some control over his circumstances. It is part of the twos, though for many littles it starts around 1.5 years. Keep showing how YOU share and use good manners, try to be understanding that this is a stage your little guy can't really help because he has no perspective yet, take deep breaths as needed, and remember to laugh often.
It may help quite a bit to agree with him enthusiastically that "Yes, that's yours!" Then you can gradually clarify distinctions between what is his and what is not. Lots of repetition is needed – it's a learning process, just like everything else.
The throwing is actually a physical/mental developmental stage, too. That repeated overhand motion is something kids in that age group are compelled to do, and it helps them build eye-hand coordination. Work on making it a game by giving him appropriate throwing toys, preferably soft, and an easy target like a box or basket. A foam bat and targets he can hit at with overhead swings may also help him get there happily. Any time he throws or bats inappropriately, you can distract him with one of his permitted toys and cheer him on.
Preparation will help you enjoy this stage. He'll be on to other things before you know it.