I have not personally done this as my daughter was born here, but as it happens a good friend of hers came over from the US about this age. I think it works best if you can manage it so he starts in Year 7, where there is a lot of moving and mixing anyway, rather than in Year 6, where people may be less interested in making new friends. I can't really comment on how the transition has gone for them, although I know the friend was quite homesick. But she has made plenty of new friends and to the outsider it looks like she has adapted very well. And kids find it quite interesting that she is American. Of course everybody's experience will be different, but I would recommend trying to time things just right.
Good luck to all of you.