My son is a single parent in the Army.
He left my grandson with me when he was deployed to Iraq. When he returned, he refused to take back his child. My granddon has been with me for 4+ years. He is seven. His dad visits him 3 times a year and only pays daycare, no child support. I filed for temp custody. My grandson was admitted to a psych hospital for two weeks and when he was discharged his dad came and took him to Wa. I live in Ut.
My grandson is Autustic and has ADHD. He has had 8 major surgeries in the last 4 years and his dad has never been there for any of them.
My grandson calls me mommy and suffers from major separation anxiety.
What should I do? My grandson begs to come home. This home is all he's ever known.

You say that you filed for temporary custody. What was the result?

Your question is rather confusing in that you first say that "His dad visits (present tense) him 3 times a year", but then indicate that your son took your grandson to Washington.

As the father of your grandson, your son has custody and there is really nothing you can do unless:

you do have a custody order giving you custody as opposed to your son; or
you can allege that your grandson is in danger of abuse or neglect at the hands of his father. Separation anxiety will not suffice, in my opinion.

Note that should you need to file suit for custody, you have only six months since the time they left Utah for Washington.