Are you thinking of secondary in UK? In which case, booking a trip and seeing several schools would probably be worth it.
In terms of winning a music scholarship, you just cannot be sure. Firstly, if the cost of the school is a consideration, check who value of the scholarship. The amount of the fees might be very small, or purely honorary, though there is usually tuition in one or more instruments included (which is in itself valuable if you'd otherwise be forking out for that on top).
Your DS would need to meet the academic standard of the school (ie needs to pass the entrance exam, but only needs to pass not reach the higher academic standard). He also needs to interview well, which means he needs to be able to talk about a number of things confidently. Coaching is often counterproductive, but if you do things like ensure he reads a newspaper most days (including features as well as current affairs), and has thought about which book or hobby he would talk about and what he could say about them. Talking confidently to an unknown (but friendly) adult is a plus - eye contact, fluency, handshaking etc - all create a good impression too.
And of course for music he's need to talk extensively about music - history, genres, notable players and composers (especially those related to his instruments). G5 for theory is fine - loads of people stop once they have secured that (to take the higher performace grades). Do work on sight reading though.