As his degree interest is relevant to the BTEC course, I don't think he will have a problem, providing he meets the entry grades. To put your mind at rest, check those universities entry criteria online, or have a word with the Admissions staff there.
The advantage with A levels is that they keep lots of subjects & career routes open. So, if he's not entirely sure about Music Technology as a career, he could keep his options open. Music Technology is available as an A level, and combining it with Music, plus 2 other, preferably academic subjects, would keep more routes open. If he were to choose Maths & Physics too, for example, he could apply for a multiplicity of technology, engineering, computer, finance degrees etc etc.
Of course, you're right. Motivation and interest is everything: not only about the potential end grades, but even if some students finish the course. Does his school have a Careers Adviser that he could discuss the pros & cons of each route with him?