The problem I see with 3.5 year olds is that they quite like to play with others, but they don't really get compromise. So five children might each be wanting to play dinosaurs, pirates, playdough, a running game and puzzles, but if no-one else feels like doing it at that particular moment they end up each doing it on their own. You can coach him in how to convince other kids to play with him, or in how to adjust his play to other's ideas.
We do co-op preschool so I get to watch the dynamics quite a bit. It's really very interesting. This year for the first time one of my kids has been in a class with a few speech delayed children, and they really do have the problem with the negotiations, but not the imagination. So they'll be doing something just as elaborate and interesting, but on their own because they can't explain the game.
