My DS started at 6 with group lesson (2 in group now!) at school through the Berkshire Young Musicians Trust. He goes to Trust local centre for a weekly "Banditos" group session - plays at concerts, competitions etc. Does your school offer anything?
Thanks milliways - unfortunately his school only do music lessons after year 3 when he will be 8. That was a good lead about the BYMT - I've done a google search on it and it's opened up lots of similar leads. Thanks.