I agree so much with this. I see he's at a school nursery and in my experience they are often stricter and have fewer staff as they have a qualified teacher.
Have you ever watched a group of very young children play with trains? They get in each other's way, they often crawl over the train tracks so it's broken (assuming this is the type used) - in other words, they are barriers to happy play for your son. He plays at home and knows how he wants to play. The other children add nothing to his fun, in fact the opposite. He doesn't share because by "sharing" he's giving up something he loves. And then he's punished.
As a preschool teacher, we were very used to children who found sharing difficult. The best way is to model and as they don't appear to be doing so at nursery school, you have to do it at home.
"Now I explained to him that in school we have to share our toys and the other children also like to play with trains and taking turns makes everyone happy. I tried to implement this at home with his older siblings and when he did scream and shout wanting his own way - I found he was able to be reasonable and calm down after we spoke to him about sharing (basically kept reiterating the same thing over and over again)."
That's fantastic.
With those children who found it really hard to share, I'd take it back to basics. First, teach about turn-taking as that's easier to grasp than "sharing". At work, I'd have the child sit opposite me on the floor and we'd roll a ball to each other and say "Your turn" (rolling the ball) and "My turn". Quick and fun. Always fun. Then move on to taking it in turns to build a tower of bricks. You say "Your turn" as he places a brick, then "My turn" and so on, with "good sharing!" praise (always name what you're praising). Building a tall tower is fun!
Trains are clearly your son's high-reward toy so once he's got the idea of turn-taking and "sharing", have a train in your hand (not his favourite!). Sitting opposite each other again, say "My turn with the train" and roll it back and forth in front of you . Then say "Your turn with the train". You could count down 3-2-1 as it nears the end of his turn and say "My turn with the train" and praise for "good sharing!". Keep the turns short and make it fun.
Build up to tracks - "Your turn" "My turn" as you each fit a piece of track and "good sharing!"
Just keep building up to taking turns with the trains on the track, swapping trains around - I'm sure you get the picture. His sisters can be roped in.
At preschool, we'd build up to the child playing with a chosen child known to be calm and a good sharer. Gradually, the non-sharer would realise they had more fun playing with others than on their own and would be motivated to share, but it takes time. Other children aren't as patient and forgiving as family!
We didn't have a naughty chair, but we did have a thinking chair which was used when the child was absolutely overwhelmed and needed to calm and have some thinking time as he did so. In fact, one child used to put himself there if he was getting cross! We also had a big sand timer.
I'm so sorry that nursery school is such a negative experience for your child so far and really hope the meeting goes well.