My kid is US citizen, living and working abroad for a non US company and pays his taxes entirely abroad. how can he file his taxes?

My kid is a US citizen, living and working abroad for a non US company and pays his taxes entirely abroad. He is making less than $125K a year. He didn't receive W2, how can he file his taxes? for sure he wants to file as he has a federal student loan. Not sure if the 1040 form is the one to use.