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?
I work in a college and BTEC level 3 music is one of the courses I look after. It is far from an easy option and actually involves a lot of written work and technical understanding.
He will really benefit from the course, the UCAS points need to be generated from the two year option (I'm sure you know that) and each university will have their own entry criteria. For example, DDM, DMM...etc
My son is bright but doesn't enjoy academic work unless it's a subject he loves and so has chosen not to do A levels next year. He would be capable but miserable I think and instead he wants to take a 2 year BTec Music Technology course which he has been told will be enough UCAS points (if he does well enough) to continue to a degree course. Whilst I can see he will be much happier doing this hands on course I am concerned he may be selling himself short but it's getting a balance isn't it ? If he did A levels I think he may struggle because of a lack of motivation but will this course really give him a fighting chance of a place at Uni if he wants to go ?