Live/work overseas - how to treat salary?

Hi,  My son lived in New Jersey until August 2021 and then took a job and is living in Abu Dhabi, UAE starting September 2021. He is a US citizen and will remain a US citizen, but will be living and working overseas for the next two years.

 

Question - how do I treat the salary that he has earned in Abu Dhabi from Sept - Dec 2021?

Is this taxable in the US because he is a US citizen? Or do I completely ignore this salary because he is living/working overseas?

 

Would amyone be able to walk me through the filings? 
I would very much appreciate it.