Year 1 is still young, and children are very much in play mode (as they should be!). Assess again at the beginning of Y2 to see if he is capable of discipline, sitting down to work at something, as that will tell you more about whether the structure of your prep would suit him better than state. Music (and the practice of) is fantastic for instilling good habits and structure, so the music heavy prep will likely be a great environment for any child, even if they aren't particularly musical.
But.
Given the VAT and escalating costs of going private, you will need to think deeply about the following, besides the ever present affordability question:
- what are your goals for secondary school? Private or state? If state, is it grammar or comp? That makes a huge difference in which path to pursue. Even non selective preps in a city like London have children working at least a year ahead of their state counterparts and it's impossible to catch up without extensive tutoring.
- Character: Is your child academic, musical, sporty, a generalist? Are they very sociable or shy? Do they get easily influenced by others? How do they cope currently if things go wrong, or they don't get their way, or if they have friendship issues? How much respect do they display for authority (either yours, or the teachers). Compliant or questioning? How well does your current school support these aspects of his personality?
Does he have lovely friends that he's very happy with? Do you have a friendly social group with their parents? In general is the social group aspirational, valuing hard work and good behaviour, kindness etc. (this is more important than school itself)
Academically, if he's in the middle of the pack and your ambition is to get him higher: what does that actually look like in practice? What level do you want him to be at and how will that be measured? And what do you hope that level to attain for him? (I.e. entry to some kind of selective education?). How much work are you willing to put in at home, in doing worksheets, support, enforcing daily reading and music practice etc.
People choose private for a whole host of reasons. For us, it started with trying to meet oldest child's academic needs. Prep absolutely achieved that for us, and has been great for our other two non academic children too. DH and I both work full time. Our choice was to either (a) have me give up work or (b) have a nanny that took all three children to various activities after school and managed homework and tutoring, or (c) we keep them at school for clubs, music, sports and aftercare for the full day, and each child gets to do whatever activity they like, and not have to piggy back on a siblings preference. We chose the latter in order to simplify our lives, and that has been hugely worth it, including for the prep and exam support our oldest child got for the 11+. May be a completely different equation for your life.