Do you have a medical diagnosis for autism or ADHD or just school diagnosis?
A “developmental delay” is only eligible for special ed under that category until a kid’s 10th birthday.
If you can, getting a medical diagnosis for autism makes other services available even outside school (see kidswaivers.org). Doing so can be difficult both in cost and in the backlog of neurobehavioral health clinics that do autism diagnosis. You would probably need a referral from the kid’s GP/pediatrician.
You can get more services on an IEP than a 504. If your kid needs individualized instruction they need an IEP. Either way your kid will have special protections when it comes to disciplinary action. A school can not academically disciple (suspend, expell, ot remove from the classroom, or e.g. not allow to attend full day vs half day) a disabled student for more than 10 school days in total over the course of a school year without holding a “manifestation to termination meeting” to determine if the behavior is a manifestation of the disability. If it is the school has to implement behavior intervention programs instead of disciplinary action.
If they ask you to come pick up your kid early that constitutes “suspension or removal from the classroom”. If that happens keep track of the number of days, and use those words talking to administrators.
I don’t know if those rules apply to preschool and kindergarten, but they should if kindergarten is mandatory in your state.
School district are also required to attempt to identify children with disabilities living in their district prior to enrollment in a process called “child find”. This is to provide early intervention programs, like developmental preschool.
It’s extremely distribing that your son was excluded from preschool due to a disability. I would be considering talking to a lawyer if I were in your place, at least seeking out the advice of a Michigan specific advocacy group.