Why did they allow him to do Maths A level with a 6 at GCSE? That nuts. That's failing him from the off. He should have been given better advice.
Re music production, it's really really hard to get into. There's few jobs and it's really competitive. University courses are not necessarily the way forward because it's an industry that's all about experience and who you know. It's one to get off your arse and graft. Volunteering to help with anything you possibly can is probably worth more than an A level in maths.
This is definitely one where you need to look in multiple directions. University may be a gateway in terms of opportunities available whilst studying more than the study itself ironically.
I know quite a few people who work in music production. A couple have made it through being quite well known professional musicians then investing the money into their own businesses and using the social network they built up from that. Another was a fan of a large band. He made friends with the band and the crew and went from there. I think he did go to uni to do music production and actually dropped out. He's doing ok, but I know he had to supplement what he was doing for very little or no money working insane and unsociable hours in bars. Then the other one I know was more academic but again it was down to work experience whilst studying and he was one of a tiny number of his course who managed to get the gig straight out of uni as there just aren't the jobs out there.
You essentially have to create the jobs and opportunities.
Honestly if he wants to go into the industry, it needs to be stressed how much he has to fight for it. The decision over maths and then his subsequent attitude to it, makes me raise eyebrows in terms of that. It has to be your life passion and for it to be almost all consuming rather than a course you do at university.
What outside his course is he currently doing related to music?