Can my husband apply for citizenship if he broke continuous residency 10+ years ago?

My husband has been a US resident since 1993 when he was 17 years old. We were checking his passports etc to fill out the citizenship application and noticed he was outside of the US from 1997 to 2001. He has never been out of the US longer then 6 months before or after that and since 2011 he has been in the US continuously. He has been in the US enough time to apply for his Citizenship but we are now unsure if he should apply and maybe get his residency taken away? We got married in November of 2012 and we are planning on moving over seas this summer for work. We are unsure on how to proceed. Should he apply and be truth full about the issue and hope for the best? Should we not risk it and just keep his residency? He wanted to apply for citizenship because we both want to move over seas permanently, we are planning on starting a family etc. and don't want the added expense of him having to travel to the US several times a year. Any guidance is greatly appreciated!