Derek the only issue is that some comprehensives (who are happy to take you for Year 14) will need you to do 3 A-levels in order to get the funding for the place. It will be completely free though. Doing one A level retake at crammer shouldn't be as expensive as a year in a boarding school, I think it works out at £4,000 possibly? And of course at a crammer it is a flexible day, you just go in for the relevant classes (I Know this I retook 2 A levels in one year, one syllabus from scratch) Try googling MPW, they are the expensive end of retakes but they might have compararive info for different nos of retakes in one year.
I have friends' who children bumped up their grades in Year 14 without going to school, just from their own efforts in the garden shed (Chemistry and French respectively) They went to a public examination centre and entered privately (small cost to enter)
I wouldn't give up hope, if he wants to go to uni he might either find a uni that doesn't ask for such high grades OR he will want to get higher grades to go the uni he wants to go to...this again happened to friends' sons...And if he doesn't want to go to uni, well it is not the end of the world..there is the rest of his life to decide to go into further study..so many options which do not involve uni at his age. (as I repeat to myself rocking to and fro, my dh did not go to uni until he was 24 years old after working for 5 years - and then he went to Kings and did History and got a 2.1.)
My problem with ds is that he definitely will not retake or get a job, he would rather just go to the unis that will take him (and some unis will take you with just two A levels and a Grade 7 (which he has)