It's possible that his teacher is comparing him to other 6 year olds in the comments but the marks reflect his performance compared to his classmates. He may be exceptional for a 6 year old but he probably has exceptional 8 year olds in his class whos parents have chosen not to advance them - it's unlikely that he could achieve the same grades as a gifted child who is 2 years older.
I hope i"m not overstepping the mark here but do you think that his reaction is more about you and his perception of your expectations than about him? In each of my schools there were a few students for whom report card days were the worst 2 days of the year because of their parent's expectations, despite the fact that they weren't bad students. Most of my report cards said 'could try harder' lol but I can confidently say that I enjoyed education far more than they did. The brainiac who won the scholarship in my year got into medicine and then dropped out when she realised that she didn't have to spend her whole life living out her parent's dreams. These days she works in a bar in Fortitude Valley.
I truly believe that our success and and more importantly our happiness is based on our EQ far more than our IQ so report cards aren't the be all and end all and we never need to expect our children to bring home A's.
If report cards are making your son unhappy why not rip the next one up and throw it out without reading it. If there are any issues you really need to know about they will let you know.