You can keep them back from prep, however unless this is decided by teacher/support worker etc, you will have no choice but to start him in grade one the following year. The prep teacher can choose to hold your child back from starting year one if necessary, but if you don't enrol them in prep, as it is not compulsary, you will not be able to enrol them the following year - it is all based on birthdays unfortunately. This is in Qld, other states are different.
I wasn't aware of this rule until recently when talking to a support teacher about this exact thing.