Hey OP MFL is my subject – not Spanish really but I can maybe help with some key things.
If he got a 5 at GCSE – do you have any analysis of why? I am thinking specific papers where he dropped marks, showing a weakness in that area (speaking, listening)?
If not, then the key elements to work on are usually verbs and vocab. There is a big step up from GCSE to A level and there will be an assumption by teachers of a certain level of skill. In this sense it is more like maths than (say) history, which is topic based. If you came into maths A level unable to do algebra, you would struggle as it would not be retaught. Same kind of thing with MFL.
So he needs to go back to GCSE content and make sure he is really secure, which I imagine with a 5 he isn't - so it's bound to be a struggle.
Go to the AQA website and find the GCSE higher vocab list. He needs to be really confident in that. So can you (or he) break it down into topics, tick off words where he is secure and then learn the rest – 10 words a day or similar?
Then verbs – he needs to have all the tenses at his fingertips. How is his knowledge of past and future tense, regular and irregular verbs? Again, get a grammar book or just look online (or a decent dictionary will have a verb list in it) and check which ones he knows then learn the rest.
There is sadly no easy way to do this – it’s a case of learning all this stuff and there’s not a quick video tutorial that will sort it – though watching Spanish films (especially the one he will study!) with subtitles won’t do any harm. How does he revise best – audio to listen to on his phone on the way to school, post its on the bathroom mirror, you testing him, working with a friend?
Do you know which film and book he will be doing? I advise buying the study guides (not expensive) as they are invaluable.