DS2 started up here at 4 and 7 months. We dithered about deferring him, particularly because DH is Danish and feels that his children shouldn't be going at all until they are 7 (!), but eventually he did go.
We eventually decided to send him so that he could stay with the friends he had made at Playgroup. In a small village, he knows everyone who is now in his class and would have been gutted not to have gone to school with all his mates.
It is sometimes worrying when he is invited to a six year-old's party before he has even had his fifth birthday, but these have been his friends for years. Would your son know many people in his class? I think it also helps that DS2 is tall for his age, so he doesn't stand out, iyswim.