Hey @Mabs49 yes you do sound a bit anxious tbh.
Firstly, like others, I assumed your Ds was going into year 9 or maybe even year 10 (bit late to choose GCSEs but...) - unless his school chooses in year 8 you all have time.
A lot of people posting here clearly loved (or their DC loved) music GCSE - which is great, of course. It sounds as tho that might not be your DS tho. He needs to choose what to study tbh. I infer he will be taking French anyway - so does he want to do music or German, or is it that you think they will be easier than other options? The best option (once the basics are covered, by the compulsories) is what a student will enjoy.
I terms of language, yes, time spent in the countries will improve his language skills, for sure. But will he enjoy it? If not then you risk making the whole thing a burden. Would he maybe prefer to take DT (or CS, or RS, or drama, or art)? That would be fine.
I worry a bit about your analysis of why your DD dipped from 9s to 8s in some of her GCSEs. An 8 is an amazing grade. Eight years ago that would have been an A-star and everyone would have been delighted. Even Oxford uni does not discriminate between 8s and 9s at GCSE. Her extra work on her speaking exam is not what "cost" her those 9s - nothing cost her them, she achieved amazing 8s.
I know that students do panic about the speaking and of course I see why. And I know that for some, that will translate to a lot of learning of answers. That's OK if it helps them but in the end the speaking is only 25% of the exam overall. Your DD did so well to gain a 9 and an 8.
As a side note - @redskydarknight interesting that a colleague's DS got a 7 but is bilingual. Is he actually bilingual? I ask bc I have had students sold to me as "fluent in German" and it is not always the case. If he is native speaker standard, then he must have been poorly coached for the exam, unfortunately. Yes you need to do what you need to do - more so for A level but even for GCSE you need to (for example) develop your answers in a way that might seem odd to a native speaker. I have ranted on about this on MN before (I do speaking tests as an external examiner) - native speakers often think they can rock up to the A level exam and just wing it. I'm a bit shocked he was so badly let down at GCSE tho - did he have lessons in the subject or was it externally sat? Did he have any kind of tuition? Even a couple of hours would probably have been enough to secure a grade 8/9 at GSCE.