To put grandson on insurance, make health decisions, can I just have power of attorney, or do I have to have guardianship?

Neither will in itself enable you to carry your grandson on your health insurance. Whether he needs a guardian or can grant a medical and a durable (financial) power of attorney turns on his legal capacity.  Some people diagnosed with autism are high functioning and can grant a medical and even a durable power of attorney.  Some require a guardian. Different states require different medical evidence.  You may want to consult a special needs attorney who handles guardianships.  To find one near you, please see the website of the National Academy of Elder and Special Needs Law Attorneys (www.naela.org).