My DS was in a similar position with a very 'late' diagnosis of autism - by which I mean he was diagnosed in the December after a two year wait to be seen. I pushed very hard for him to delay starting school that year. He was born July so was just 5 before starting school.
By the time he'd had another six months of nursery he was bored and ready for school. One of the concerns from the council was 'the other end' of things - that he could leave school at the end of third year - and again (now we're at that point) I can see that they were quite right.
School support in Scotland is patchy, getting it right for every child is a pipe dream in many situations, but I believe that most of the time the age rules for starting work in the child's best interests in the long term.