Hi @GCSESbegone, I'm not going to get into how grades were done in my school, as I think that is a separate thing to what you're actually worried about.
His targets for A level outcomes (not predicted grades) will be devised from his GCSE average point score (this is what the DfE use to measure progress in KS5) and set accordingly.
These are a target not a limit!
If he is genuinely good at computing, then I am guessing that he spends his own time writing programmes, making games/apps etc? This is what will give him the understanding and experience to handle those parts of the course. How is his maths? If he got a high grade in his maths, he will likely be absolutely fine (and his teacher also said so!).
Computing is probably the 'hardest' A Level under the new reformed A Levels, but that doesn't mean he won't do well. Also remember that students moving between schools at this point and doing A Level computing may not have had the opportunity to take GCSE computing, because many many schools just can't offer it. Their maths knowledge and understanding is what is crucial though,as you do need to be able to think logically and creatively to devise programmes.
His UCAS predicted grades will be based upon his work/assessments in Y12 - his teachers won't even look at his GCSE results when devising them, in all likelihood. That is what is crucial- that he gets into a studious mindset, works well this year, revises and performs well in his internal exams next summer.
I understand how upset he would feel if he's strong in the subject then got a poor grade... but it's just one tiny piece in the jigsaw that is his whole academic career, and it really will count for naught once he has his A Level results. (And, yes, I know a very few universities demand straight A*/8s at GCSE for certain courses, but there are hundreds of institutions and thousands of courses, let alone those in other countries!)
If the teacher is happy with him taking the course, he should stop worrying and borrow from his teacher's belief in him. Best of luck to him!