It worries me because at home, he memorized about 12 nursery rhymes by age 2, counts to 40, knows everyone's names in class, many of the states in the United States, collects bugs, and organizes everything. But at his school, he doesn't show any of this. Should he be in a smaller environment, where he feels comfortable demonstrating his talents?
I really don't think that the classroom size is daunting to him. There is likely a teacher's aide in there too. I am quite familiar with the Montessori system. Ask the teacher what she is doing to encourage his participation and interaction in the classroom.
At home have your son play act some possible social situations with you. Example, he wants to play with a toy another kid has at school. Practice with him how to approach that child, what to say, and what speaking voice to use. When he is happily playing with his buddies at home, gently remind him afterwards that this is what you would like him to do at school. Are there school peers who you can have over to your house? If he is successful in small play dates with school friends, this may translate over to classroom interaction.
It's too soon to worry that this will be a permanent problem. With encouragement and practice, his willingness to speak up in the classroom and at school play should increase.