am wondering what the rules are on admitting SEN children
Agree with what others have said. A statement naming the school would ensure admission - but, as you realise, statements aren't easy to get. (Statements aren't usually listed in a school's oversubscription criteria, because it doesn't matter whether the school is oversubscribed.)
Some schools have an exceptional social/medical criterion - but the threshhold is high (i.e. it is likely that evidence from professionals would need to show that no other school could cope with the condition).
Otherwise as SB3 says, it probably depends on whether you can appeal for a place by demonstrating that the chosen school is especially suitable for your son's particular needs.
Agreed - you would need to research this carefully to compile the evidence, and I would advise meeting with the SENCOs at any possible schools you identify.
To give this thread a bit more exposure I'll move it to the SEN section - via Bucks - leaving behind a 'shadow'.