So sorry that you and your DS are going through it. I consult with some companies in aeronautics and they are actually doing better than ever, and all are hiring, especially in the arms and defence sector.
If he is getting interviews, his degree, his CV, and his volunteering are working. He seems to have an "interview leak" that needs fixing. I don't know if you've answered it before, but can he get himself an interview coach? One that is familiar with the sector, because it's so specific.
What I've seen in fresh graduates time and time again: Aerospace companies aren't just looking for people who love planes and drones; they are businesses. If he can't articulate how a project affects manufacturing costs, supply chains, or regulatory compliance, he will lose out to candidates who do.
It's probably hard after a year, but if he's coming across as just looking for a job - any job - the companies will pass him over. Also, he shouldn't talk down his charity job and treat his volunteering as a side note or placeholder. Sustainability is where the entire aerospace sector is investing, he should make his volunteering there look intentional.
Looking for a job abroad my be hard, because the entire sector is tied to national security. Unless he is looking at purely civil, non-defense aviation roles in specific hubs (like the Middle East or commercial aviation hubs in the EU like DHL in Leipzig), looking abroad as a fresh graduate is an uphill battle. He will likely get a much higher return on investment by fixing his UK interview strategy.
Is he is only applying to airlines and aerospace primes (like BAE Systems, Rolls-Royce, or Airbus)? I would advise him to look a bit further, he should try applying to wind turbine manufacturers (Vestas, Siemens Gamesa, ,ORE - they are hiring like crazy), engineering consultancies like DNV, Wood PLC, Mott MacDonald, automotive companies (they have all stopped hiring, but it's still worth it to try JLR or WAE), or rail & infrastructure companies.
He needs to tweak the language on his CV and in his interviews. Employers in these fields sometimes worry that an aeronautical graduate is too niche or just wants to work on jets.
I wish you luck!