yes - for the Saturday music courses (like Junior RAM, Junior RNCM etc etc) the fees are about £3000 - and if you want a bursary, you must apply and then any scholarships will be based on your income. If you are over the magic MDS scheme number of £59,000 (?) then you will probably pay full fees.
Tuition for the specialist music schools, (Purcell, Chethams, Wells, Menuhin or St Mary's), is sliding scale (based on your income) IF your child gets an MDS grant (which is based on two auditions). If you around the £57,000 mark then you might not pay very much (the fees are usually listed on websites of these schools). HOWEVER - once you turn your kid over to one of these schools, you don't have much say in what's happening with his/her music and sometimes the academics aren't very good.
Some music services are excellent - but I'm guessing that Northumberland has had loads of cuts. So - you are correct teabiscuit - it's really tough out there...and bloody expensive too. Probably one of the best things you could do is to get him through his grades with high marks .. this lends credibility to any sort of financial requests you might make based on his talent. This also means getting him through the grade 5 theory and working hard on the aural portions of grades 7 & 8 - cuz they aren't easy unless you have perfect pitch. (And if your son is already so advanced then it is quite possible that he DOES have perfect pitch - do you know?) As for NCO - this isn't necessarily everything it's cracked up to be. Have you looked at ProCorda North? this could be really enjoyable for your son...
in terms of private (independent) schools, then it's quite possible that he could get a scholarship from an independent ... if it's a local (to you) place, it would cover his music lesson fees. If you send him to a boarding school - there are quite a few that have excellent music programs (Uppingham comes to mind.. but there are many more) and would probably give you a lot of scholarship for a grade 8 fiddle player... so - get that Independent schools handbook - and start checking things out.
Music education is an investment.... it will enhance your child's life when he is older... one of mine has started uni - and absolutely loves the orchestra and other musical activities that are open to all of the students. It was the same for dh - he says he would have had no social life at uni had the orchestra not existed.... life is different and better when one has these 'special' interests... don't give up on helping your son!
Best of luck!