It is NOT normal for a 2yo to know all his letters and be able to count. That, coupled with some speech issues leads me to suggest you get him tested for Autism. The pre-k teacher is usually trained in seeing these developmental issues and you would be wise to listen to her. He may just be exceptionally smart.
This is not a crushing issue and the earlier you get therapy, the better off he will be in the long run, if he is on the autism spectrum!
I whole heartedly DISAGREE with Butch and the other posters who say it is not the teachers job to make observations. While it is not the childcare providers job to diagnose, this is NOT what the childcare provider is doing. She is relaying to you, the parent, some concerns of hers- very appropriate and her job. As a former, highly qualified, childcare provider, we often see things the parents do not. We are trained in child development and how to recognize when to inform a parent about something that is not quite right. It is our job to bring these issues to the parent. She is making an observation and letting the parent know- doing what is in the best interest of the child. What the parent does with that information is her own business.
Testing is often free, through ECI or the school district. It is better to suspect and go through testing than to ignore those little signs and let it go beyond effective, early treatment.
Go with your gut. If you feel he needs to be tested, get him tested. If not, work with him on your own. Say words properly and slowly and let him see your mouth and tongue movement. Often, little boys are slower to develop speech than girls, especially if they are younger siblings. My middle boy did not say one word before he was 3 and then, boom- full, complete sentences were coming out of his mouth.
Good Luck!