I am not sure what the school district policies are for children who are not yet toilet trained. However, if your child has emotional issues he will likely qualify for a 504 plan. There is information on a 504 plan here:
http://www.chtu.org/504.html

The goal of the school district is to place the child in the least restrictive environment. If this is your child's only issue, the least restrictive environment is obviously a typical kindergarten class. I'm not sure what arrangements can be made, but there is no reason to place him in special education.

As far as encouraging his progression, this is obviously an emotional issue (_base_d on the fact that he was fully trained before). Keep dealing with incidences calmly & without shame, and if it's "just" regression it likely will resolve as the child settles in. If it's a control issue (common in children with attachment/bonding issues) that's a different matter.

You mention your child has only been with you a short time, I assume he is recently adopted, or a stepchild? I specialize in post-adoption support and attachment disorder issues. You may want to carefully consider whether your child is emotionally ready to begin kindergarten with his same-aged peers. Kindergarten is optional in Utah, so he could be kept home a year and start first grade on schedule. You may also want to look at private kindergarten programs.