You might have already tried this also but Nippers down at the Surf Club is a great way to meet friends of his own age.
My kids, aged 12, 10 and 7, all love it and the baby will start in around 5 years time.
Unfortunately the season finished at the weekend but I am sure any of the surf clubs on the northern beaches would be more than helpful if you pop along? We all belong to South Curl Curl http://southcurlcurlslsc.org/
Chris