I'm mulling it over in my mind because it hadn't occurred to me before, so I would value some outside opinion.
Background: DS has ASD and attends mainstream school with a 1:1 SNA and the maximum resource hours. He can be quite disruptive, impulsive and argumentative/aggressive, but he can also have good days/weeks when life seems easy for him (although it's not very often). He is coping reasonably well academically. He is very immature, this is often commented on and I can see it myself. His understanding of and reactions to various situations are that of a much younger child.
We are in Ireland, where there is more flexibility with ages and children tend to start school when they are ready and it's not unusual to move up/down to find the right level for the child. They spend 8 yrs in primary (Junior Infants, Senior Infants, 1st Class to 6th Class) and then 6 years in secondary (although some miss 4th year which is called Transition Year and is not academic). The secondary school that DS is most likely to attend has a Prep class, which is pre-1st year and usually instead of doing 6th Class at primary school.
What the psychiatrist has suggested is letting DS finish primary school and then go into Preps for a year before 1st year. So he would be doing an additional year. There are some downsides to this, mainly that quite a few of his classmates will be going to the same secondary so he will end up a year behind them and having to make all new friends (although he is fairly extroverted so it might be ok).
What's throwing me the most is that he will be really quite old when he finishes school. He has a June birthday, when we moved back from the UK we put him down a year because he had always struggled with being one of the younger ones, and it really helped. If he stays back another year he will be 19 when he enters his final year of secondary and he will turn 20 during his Leaving Cert (final exams). He is likely to be at least 3 or 4 months older than the next oldest child, and well over a year older than the average (when I was at school the 'average' child turned 18 in about January of 6th Year).
He is currently in 4th Class, so we would need to let the secondary school know next year (if they are willing to allow it).