Question:  how can a spouse of USC receive green card if he entered the US illegally more than 10 years ago?

Answer:  In order to receive green card (adjust status) in the US, the alien must generally enter the US legally (even he or she later overstayed).  The exceptions to this rule are the 245i law (enter the US before 12/20/2000 and an immigrant visa petition or labor certification was filed for him or her on or before 4/30/2001), cancellation of removal and asylees.  If your husband does not qualify for 2451 or asylee, he should apply for cancellation of removal, which require the proof of extreme and unusual hardship to you if he is removed.  This application is very complicated and I suggest your husband consult an immigration lawyer since he is in the US.