Is he going into Kindergarden or first grade. Leap from Letter fun factory DVD is great!!.. Help him at home..Their minds are like sponges.. It could be just the teacher.. Like my son had in 4 year old pre K... wanted to hold him back.. I sent him on to kindergarden. They need to keep with kids their own age.
I definitely think it is important to not get discouraged. My son just finished K this year and I was told that they advised testing because he wasn't up to speed with his peers. I was so distraught, thinking that he would always be behind but after testing, it was concluded that 1. he was the youngest in his class (his bday is during the first week of school)
2.he has no learning disabilities or problems.
I think that your son will be fine. He may just need to adjust and you may need to figure out the learning style that works best for him.
I also think it was a bit premature for his teacher to say he was behind on the first day. I'm sure he is already doing better and catching on. He probably just needed to get comfortable with the work along with the social atmosphere that accompanies it.
I would be concerned that a teacher was trying to make assumptions about your child's ability on the first day of school. Many students will not present in their current ability or personality the first week, as they are still getting used to their surroundings and new environment.