Does he meet the Substantial Presence Test?
https://www.irs.gov/publications/p519#en_US_2025_publink1000222127
If he does, the 'default' treatment is he is a full-year Resident unless you establish and file a statement for an earlier termination date.
https://www.irs.gov/publications/p519#en_US_2025_publink1000222182
