I am doing my son's taxes. He is a college student in Az, worked in Az, but we live in CA. He did not work here. I have a prompt for Form 309 inc subj to tax by oth st. ?

[Cynthiad66]

We will need clarifying information since you did not really ask a question.  But, from your statement it seems your son is filing as a CA resident.

If California is his home state,  you will need to file a CA and AZ tax return.  California will tax all income whether earned in California or not.  

Form 309 - AZ form Credit for Taxes paid to Another State
Use Arizona Form 309 to figure your credit for taxes paid to Arizona and another state or country on the same income. If claiming a credit for more than one state or country, complete a separate Form 309 for each state or country

Arizona Form 309 Info