The only thought I have is a montisorri school - I know of ones from kindy age to early primary - they are different to 'regular' school, but not sure how many of them there are for you. Childcare centres are allowed to have prep aged children attending, however that doesn't solve your issue with wanting to start him later for prep. Perhaps talk to his kindy teacher, they may be able to assist you with having him delay his prep start - you can do it if you have a supported reason as far as I know.