If you are completing his 2022 tax return then his resident state is where he lived and maintained his domicile as of 12/31/2022.
For tax year 2023 that is filed next year you will have to determine if he has changed his domicile to your state. And if he has then his resident state for 2023 will be your state assuming that you plan to have him stay with you on a permanent basis or live in an assisted living community near you in your state in 2023.
An individual generally has only one domicile, which is the place considered the true home, the place where the individual intends to return to when away.
