I think he just needs to adjust.
Your question and the Teacher's response seems reasonable. They are trying and validate your concerns.
That is good.
They are trying to help him. They show compassion.
My Daughter, was a bit like that, like your son. She was seemingly 'shy' with her class... BUT it was because she is the type that is not instantly gregarious... she OBSERVES first... THEN actually CHOOSE who and what to play with. She will speak up though, for any right/wrong things done to her. We taught her that. But she is a very self-reliant child... and was able to KNOW herself... and we didn't expect her to be SUPER extroverts like the other kids. She is herself. She adjusted fine. She found friends, she got along with. NICE kids. She observes and chooses well.
It was fine.
Some kids a just that way.
But at home, our girl was SUPER extroverted and outgoing. Of course. Like most kids.
She was not "unhappy" at Preschool... she just took time, to adjust. But she actually LOVED it.
Did your child ACTUALLY say, he is "unhappy" at Preschool??? Or do you just think he is???
BUT, the Teacher, can ALSO talk to the WHOLE class, about proper behavior/sharing etc., and how to be polite. THAT is what my Daughter's Preschool Teacher did. NOT just talking to my daughter about "her" behavior... and how to come out of her shell.
A Teacher, ALSO has to address the WHOLE class, on behavior too... and ESPECIALLY if there are strong personalities in the class because these kids, can literally take over and command the other kids.
SO... maybe suggest that to the Teacher, as well... that she talk to the whole class about proper 'manners' etc.
all the best,
Susan