Hi M.,
He's still really young and these are social skills he is just know starting to learn, so he isn't close to mastering them. It's okay, because most other kids his age haven't either, and lots of times the ones who seem like they have, seem that way due more to personality traits than a set of skills.
Things you can do to introduce and reinforce social skills:
1. Model what it is you want him to do. Say "please", "thank you", "excuse me", "may I", "yes/no sir/ma'am". Offer to share what you have with him and so on.
2. Engage in practice play with him. Sit in the floor with him and play with him and encourage the way you want him to play with other. Use positive reinforcement: "you are sharing so well, when you play like this you will make lots of friends". Ask him to share with you and you share with him. Explain the "rules of sharing".
3. Roll play with him how he can act with other friends using dress up, action figures, or puppets.
This is one of those places in their learning that practice makes perfect.
Really go over the sharing guidelines with him frequently: Sharing means taking turns, giving some of what you have if you aren't playing with all of it, playing together, and WAITING your turn. Just because someone has something he would like to play with doesn't mean they have to give it to him or vice versa. Someone has to wait their turn and can sit quietly, ask to play with or go find something else while they are waiting. That's the trickiest one, so that one will take lots of preparation.