many thanks for the advice...!
I have received other information and some people have stated that year 12 is the most important year, regarding scores! . My son is just under 6 foot tall and very very mature, great academic child. If we wait here in Ireland until he finishes his A levels that will be a further 3 school terms and he will turn 18 the year that he finishes. The prospect of waiting until then is not visible and then you run the problem of visas!
It is a very tricky situation but when we make the move we are in it for the long run as he would like to attend uni, and we cant chop and change as the impact on the kids is so much!