Advice? DS won't talk to his teachers.

It's DS's 3rd year of preschool (he misses the cutoff for Kindergarten this year, so he is doing the 4 year old class for the second time) and he still very reluctant to speak to his teachers.  He will answer questions with nods and shakes of his head, but will not verbally speak to them until close to the end of the year!  He is shy in general, but doesn't have a problem talking to his friends and will even stand in front of the class to present during show and tell.  But if a teacher asks him a question, he just stares at them.

I've tried role playing at home. For example, he really wanted to know if the class will have Pajama Day this year (it's a different class from last year, so new teachers).  He said he would ask the teacher if I went with him.  So we practiced at home and on the way to school.  And when it came time to ask, nothing.

This isn't something the teachers brought up as a concern. They seem to understand his nature and said he is content in school and very well behaved. But I was just wondering if there was anything I could do to help him feel more comfortable speaking to his teachers.