It is never a good idea to bribe kids, Smow - although you have EVERY right to be very proud. My kids had friends who were being offered £10 or more per subject passed at 'C'. I declined to join in; they were doing this for themselves, not me. They both did extremely well and went to Uni.. I bought each a celebratory meal after results day.
I remember a kid (when I was coaching athletics) he was called Daniel and was faster than anyone else around over a sprint. He was 10. His father gave him £5 every time he won a race - easy money for him. He then moved up an age group and, of course, he lost. Instead of buckling-to, like the others, training harder etc..... he gave up. A valuable lesson.
I think you are stuck with keeping your word! Let him work for his 'A's' for his own self-esteem. Meantime, fork out and glow. :) x