Based on your information - he is self-employed and is treated as he runs his own business. He has business income and business expenses.
The business income and expenses will be reported on the schedule C - http://www.irs.gov/pub/irs-pdf/f1040sc.pdf
The net income (after deductions)will be reported on the form 1040 line 12 - http://www.irs.gov/pub/irs-pdf/f1040.pdf
If the business has net income over $400, it may be required to file Schedule SE, Self-Employment Tax and net income is likely self-employment income and 15.3% self-employment tax would be required (13.3% in 2011).
Self-employment taxes from schedule SE will go to the form 1040 line 56 - http://www.irs.gov/pub/irs-pdf/f1040.pdf
Also - he will deduct half of self-employment taxes on the line 27.Generally - that all he needs. Please review all possible deductions - that might help to reduce tax liability.
If the same income is taxable abroad and in the US - he may claim a credit for taxes paid abroad - so the same income would not be taxed twice. Use the form 1116 - http://www.irs.gov/pub/irs-pdf/f1116.pdf please find instructions here - http://www.irs.gov/pub/irs-pdf/f1116.pdfThe credit is limited by the US tax liability on the same income - the form 1116 is used to calculate the amount of credit. Means - if tax liability abroad is higher - there will not be US taxes on that income, but if tax liability abroad is lower - in the US you will pay the difference after the credit will be applied.
To report transactions in foreign currencies - he should use the currency conversion rate on the day when the transaction occurred.
Here are government and external resources you may use for currency exchange rates - http://www.irs.gov/businesses/small/article/0,,id=130524,00.html
Let me know if you need any help.
Thus if you are single - you are not required to file your tax return is your income is below $9500 (in 2011).
Report the gross income of 4,000 with expenses of 1200 on Schedule C.
As long as he has a ss # XXXXX should use that to file.
If he pays tax on these earnings to Poland he gets a CREDIT on his us tax return.. Otherwise there is no issue.
I will monitor this as I see you are responding quickly.