A significant ex of mine couldn't read and write until 12 and was written off by his primary school. He went on to get 3 A's at A level and then on to Oxford. He was VERY bright.
Different kids flourish in different environments.
There's also a bit of switching about in Year 7. For example, I've had some very able children put in my less able classes at the beginning of Year 7 (based on primary data etc), and had to get them moved up because it was so obvious that they were in the wrong class (for their abilities).
Do you believe him to be 'average' or is this what his school is saying?