Since your son is working under OPT, I assume he has already graduated from college and that he has been living in the U.S. for more than 365 days.
If that's the case, there is something in the U.S. tax law called "Substantial Presence" test. Click on this link to see if your son qualifies. If he does, he files a 1040, not 1040-NR.
Substantial presence test | Internal Revenue Service
You will be considered a United States resident for tax purposes if you meet the substantial presence test for the calendar year. To meet this test, you must be physically present in the United States (U.S.) on at least:
- 31 days during the current year, and
- 183 days during the 3-year period that includes the current year and the 2 years immediately before that, counting:
- All the days you were present in the current year, and
- 1/3 of the days you were present in the first year before the current year, and
- 1/6 of the days you were present in the second year before the current year.
Example:
You were physically present in the U.S. on 120 days in each of the years 2021, 2022 and 2023. To determine if you meet the substantial presence test for 2023, count the full 120 days of presence in 2023, 40 days in 2022 (1/3 of 120), and 20 days in 2021 (1/6 of 120). Since the total for the 3-year period is 180 days, you are not considered a resident under the substantial presence test for 2023.
