our pre school in NSW tends to have kids from years 3-5, so there are always kids going to school, and other kids that stay behind. TBH i never really thought that kids would pick up on it? i mean, at DS's school last year before he went, some kids did leave that he was friends with and told me they were going to school and i just kind of said 'oh yeh,that's cool. well you're going to school next year when you turn 6 and that's going to be fun!' and DS was happy to stay on at his pre school.....maybe you could try that? explain to your DS 'when you turn....you can go to school.'

your DS might not even notice, though i know every kid is different.....