S.H.
Its the sibling dynamics.
I have a 6.5 year old and a 2.75 year old.
My youngest is good too, about playing by himself or sharing with his sister. And visa versa. BUT, as your daughter gets older, the dynamic might change and she might get more territorial over "her" toys or activities.
The thing is... to your son, being the "eldest"... he is probably ALWAYS having to "share" as a default, with his sister. He is always by default being the eldest, expected to act perfectly; he is by default having to "be" and do everything... in light of his sister... and it is NOT "fair" upon the eldest. AND kids do get territorial too. Its their nature. Its okay.
He also probably just wants what she has... he might see it as some kind of competitiveness or just wanting everything to be his kind of thing. He tries to negotiate with her...but she is only 2 years old... so she is on a TOTAL different developmental level, than he is.
The thing is, with our kids and in our case... instead of always making our Daughter (the eldest child) give things to her brother (ie: share) we "let" her not share. EVERYDAY, she has to "give up" her things, for her little brother... that is the natural thing to do and say to an eldest child "Sally, share with brother." or, "Sally, let him have it he's just a baby..." kind of thing. That is how an "eldest" child is expected to be and act like. BUT if you really look at it... its not fair. The eldest child SHOULD be able to have what she is currently playing with- and should not "have to" ALWAYS give up everything to the younger child. My daughter, many times, would get so frustrated because everyone would be telling her "Sally give it to brother, he just wants to play with it" or, "Sally, let him... he doesn't understand..." or, "Sally, share..." My daughter, was having to give up what SHE was playing with, all the time. Its just too much to ask a young child sometimes.
-And yes, my daughter would also sometimes want what her brother has, just because. It was out of frustration. She felt she was getting the short end of the stick. And she had a point.
Praise your son, that he is attempting to "ask" for the toy in question. Recognize, his frustration... validate it. Let him have toys that is JUST HIS, and which he does not 'have to' share. Make some toys JUST your daughter's.
I don't believe, that ALL toys HAS TO be "shared." Some toys are just more special to a child.
Next, we explain child development to our daughter... she is old enough to understand. From the time our son was born, we explained to her what a baby does, how they learn, how they react, how they will cry, how they will NOT always understand what she is wanting, how they are YOUNGER than she is... and thus, not on the same level as she is. THIS has made a BIG difference in how our eldest child interacts with our son. She understands... and does not just expect her little brother to be like her in ability or understanding, now. So, now, she will actually sound like ME, when she is "explaining" things to her brother and interacting with him. She is SO very aware now... and her own expectations toward her brother are now more accepting.
ALL siblings will or will not share. They should not have to share everything, nor ALL the time. And if they fight about it, it is a life lesson... but, you need to referee it sometimes and guide them... not just let them handle it by themselves. And if per their moods, they simply will not stop, then stop the activity. And BOTH children get the SAME punishment or alternative. For example: put ALL toys away. Not just his, or not just hers. THEN talk about it. THEN give them an example of what you expect. ie: "Johnny, I know you are frustrated and Sally is too... you both want the same toy. But that doesn't work. So I"m going to put it away now. OR, you try to take turns... Sally doesn't understand that yet and she is not like you... but lets all practice taking turns..."
Or, "Johnny, I know you don't want to share right now, that's okay. How about in 2 minutes, can you let Sally play with it?" Or, "Sally doesn't want to give up her toy, she is still having her turn. When she is ready, you can play with it, or not. She may not want to share." Or, "Sometimes we can't have the other person's toy. It's theirs. They can decide..." Then guide them on it.
From the time my kids were about 2 years old, I taught them the word "compromise." THIS has become a real foundation with them and it teaches them a lot. Kids, CAN learn "big" words and what it means. I emphasize with my kids about "sharing and taking turns..." and then we actually do it. Just for fun. We play board games, we take turns stirring something that I am cooking, we take turns watering the plants with the hose, we take turns sitting on Mommy's lap, we take turns playing with THEIR toys, and then I make sure to use verbal sentences/words that reinforce the "attitude" behind it. ie: "Wow Sally, thank for letting us play with your special doll. I'm proud that you let us take turns, and even let your brother play too...." kind of thing. We play as a GROUP. Then, when my kids want to play by themselves or with their toy only, that's fine. I respect their space. And if one kid encroached on the other... I say "Sally, Tommy wants to play by himself now. Can you respect that? Please give him a minute. He will share when he is ready..." kind of thing. THUS, it teaches them how to 'read' a situation too... and to respect feelings.
I don't "make" one child "give up" his/her toy and give it to the other... the child may not be done playing with it. So "waiting" is another concept to teach them... "wait your turn..." kind of thing. BUT, the youngest child has different developmental understanding of things.... so, they cannot be expected to act older than they are either.
Well, sorry for rambling, but this is what I do with my kids.... and they are pretty good about sharing or not (when they don't have to), and with taking turns and that their own toys are respected etc.
It takes practice, together... with you engaging and facilitating it in order to show them a tangible way of doing it.
All the best,
Susan