I did computer science last year- the coursework is time consuming (less so for your son as it's only 25% of the GCSE). The exam content isn't too hard, but it will have gotten harder since I last did it and we had an actual computer science teacher (albeit he didn't teach us everything we needed to know for the exam) and there are a shortage of those. Schools usually use IT teachers.
It is an interesting subject though, but if your son is more interested in music and is talented, I would go for that. I'm not sure about the contents of the exam but it will definitely be easier to learn.
Hope this helps.