There are lots of other threads on bursaries do search them. We've been on a substantial bursary 60% + at two top boarding school for 9 years now. When we started it was virtually unheard of now everyone knows. Also 9 years ago the economy was booming and many schools were not only optimistic for there future but also aware that the Charities Commission was looking into this issue and the general feeling was that for schools to maintain their charitable status they would have to significantly broaden access. I believe there has been a fairly ruling overturning this.
OP you need to approach this is a systematic fashion and you want to avoid raising your child's hopes until you've had some sort of positive feed back from a school.
First thing is ignore any comments like "bursaries are only offered to scholars, those on less than £40 000 PA this is not true for all schools.
Next identify two or three schools you like the look of. As a general principle the big names are more generous as they basically have more money. Boarding schools although often keen to fill empty vacancies may not be able to offer 100% because quite simply as most fees are £30 000+ thats a hell of a lot of money. READ THEIR WEBSITES usually under admissions/fees if bursaries are attached to scholarships it will say also cut off dates for applying and even size of bursary. Also do your figures be realsistic! Having identified schools you like contact the bursars say I love your school think my DC would do really well in your school, if your DC has any particular talents e.G. IQ of 160 grades 8 flugal horn, Olympic tiddlywinks player etc etc this is the time to mention it and sadly but realistically any significant SEN, explain your situation if you are going to need 75% tell him ask if they are able to entertain bursaries of this size your not asking for a definite yes but you don't want to waste their time your time and most importantly raise your DC's hopes if its a non starter.
Only after you've received some sort of positive feed back take it any further.