Once he has applied for and received advance parole, he can travel outside the US. If he leaves without advance parole, he will not be allowed back into the US.
http://www.uscis.gov/graphics/services/Emergency/index.htmAdvance Parole
Most aliens who have pending applications for immigration benefits or for changes in nonimmigrant status need Advance Parole to re-enter the U.S. after traveling abroad. Aliens applying for advance parole on the basis of a pending application for adjustment of status must be approved for advance parole prior to leaving the United States in order to avoid the termination of their pending application for adjustment
Aliens in the United States should, prior to departure, obtain Advance Parole in order to re-enter the United States after travel abroad if they have:
Filed an application for adjustment of status but have not received a decision from the USCIS;