I suggest you ask one of the moderators to move this to Bucks as you will get more information and advice there!
Good idea - I'll move it
A) Send him to local comp. Support him at home using material/notes from gs that his 2 older brothers' attend.
Is it then possible for him to take triple science? Have you done this? Could he skip doing the double/single science at school? Where would he take his tests?
science exams are currently hard to take out of school as they require observed practicals and have to write them up in class etc. Best maybe to let him do the double science with the others
Would I be allowed to teach him latin and let him take the gcse (my eldest finds this subject the easiest of all, so it might be worth giving it a try to home-school him in)?
B) Is it possible to enter gs later than 12+? eg after gcse?
yes - there is quite a bit of movement for schools at sixth form level and it may be best to find one that suits his interests at this stage
C) Is it possible to enter an indie at a later stage eg around gcse or later if we find the state school isn't right for him?
yes though better earlier rather than later - some have entry at year 9