S.,
As a child with autism, I assume that he is receiving special services through your school district that will continue into kindergarten, and then as a school aged child when he transitions to 1st grade. It would be absolutely appropriate for your son to have a personal care IEP, or hygiene IEP with toileting as a goal. If he does not have this, or has not been offered, you should bring it up so that he is using the toilet in time to be included in kindergarten, not using the toilet may be an impediment to being in the least restrictive environment for him, which will be the general education classroom as an ultimate placement. Remember, placement is not decided first, goals are determined based upon need, and placement results from that, so if you have a pre assumed placement of a special education classroom because of his toileting skills, they are going backwards. The goals should be calculated now to get him into the general education classroom.
M.