@Pianofp --
It would help if you could give us the names of the two states involved.
That's because some states have a 6-month rule regarding residency for tax purposes. Since your Dad still hasn't decided to abandon his domicile in the old state, and since he's been living with you since January, it's possible that he's now in a situation of dual residency. A dual resident is a person who's both a domiciliary resident of one state and a resident by statute of another state. This would determine how he should file his state tax return(s).
Another issue is that some states exempt social security income from taxation. And retirement income is generally taxable only by one's state of domicile.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
