Advice re moving my teenager from secondary school in Ireland to secondary school in UK?

Hi there, I am moving to the UK from Ireland in June 2017, my son is currently in 3rd year of secondary school here in Ireland. He will take his Junior Certificate examinations in June 2017 (hence why I am waiting until then). He is 15 now, he turns 16 in Sepember 2017. We both want him to continue his education to A Levels, he wants to attend Uni, he is still a bit unsure re what carer path he wants to choose. I have done a lot of research and have a relatively good understanding of the UK Education system. My question is with the info provided above where does my son stand re getting into a secondary sixth form to do A Levels, his Junior Certificate results do not come out until September 2017 and I understand GCSE results have a bearing on entry to A Levels. I am confused as to where he stands and what we need to do, I am hoping to come to Manchester. The last thing I want is his education to suffer as a result of this move. Also secondary sixth form versus Sixth form college, is there any difference? Also I want to link with a career guidance specialst for my son in the UK before we come because I want him to have all his options and guidance re choosing subjects for his A Levels, any suggestions ? Skpye consultation would be good as we are in Ireland at present. Thanks in advance LN