When you say "employer-provided" is that self-insured?
If it is not- you can pursue through Department of Managed Health Care by filing a grievance with your insurance company, allowing them to deny you and then file an appeal with the DMC. DMC rules in favor of the patient almost 90% of the time here.
If it is a self-insured plan, they are able to get around it- your only recourse is to petition your company to include the services. I would include the information that Crimson Wife included.
Keep in mind each state is a little different.

I Googled "Iowa mental healthy parity law" and found this stating that health insurance must cover psychiatric and psychological care for individuals with autism: http://www.asha.org/Advocacy/state/States-Speci...

My employer-provided health insurance has the same policy. There's a brand new Medicaid "opt in" program where, if you make too much money for other state/government services, you can pay a premium and enroll your child in Medicaid as a secondary insurance to pick up things like co-pays and services your private insurance doesn't cover. I'm enrolling both my ASD kids in it -- paying an extra $120 a month for Medicaid is worth it considering the gaps in our employer-provided insurance!

Really, you would need to talk specifically to autism parents in your area to see which service providers are a good choice.