The person may qualify for the foreign earned income exclusion - he/she should:
-- Work and reside outside the United States for at least 330 days during the year, or
-- Meet either the Bona Fide or Physical Presence tests.
To receive that exclusion - the taxpayer should file either form 2555 or 2555EZ.
Here are forms you likely need:
Please be aware that - the exclusion above will not affect self-employment taxes - only income taxes. Only earned income is excludable. For instance dividends, investment income, etc - are not excludable.
If the person qualifies, he/she may exclude up to $91,400 (2009 in foreign wages -
plus housing allowances (limited to 30% of the earned income exclusion).
You meet the bona fide residence test if you are a bona fide resident of a foreign country or countries for an uninterrupted period that includes an entire tax year. To see if you meet the test of bona fide residence in a foreign country, you must find out if you have established such a residence in a foreign country. - See more details here - http://www.irs.gov/businesses/small/international/article/0,,id=96960,00.html
I do not think that your husband is going to established a residence in Afghanistan (and it is likely that the IRS agent will not think so either)
So - to use the foreign earned income exclusion - your husband should be outside the US at least 330 days - he doesn't have to be in the same country of be outside the US for work - instead of coming back to the US - he may spend time - for instance - on vacation in Canada.
Let me know if you need help with reporting.