Accommodations are made for people with learning challenges to take the GED.  Start with the ACE website (officials responsible for administering the test).  Tests, including practice tests, can be administered by audio-cassette:

http://www.acenet.edu/Content/NavigationMenu/ged/about/faq.htm#accommodations

They will help point you in the direction of your nearest testing facility so that the proper forms can be filled out.

He will have to fill out Form L-15, which requires documentation of his diagnosis.  The L-15 is described in detail here:

http://home.gwu.edu/~kkid/f15.html

He should also contact the local community college or adult ed center offering GED classes.  By law, under IDEA, accommodations for his disability must be made in giving instruction as well as in testing.  So they should have audio-cassette materials available for his use, or be able to refer him to appropriate resources.  

It's important that he take official documentation of his condition with him when requesting accommodations.

I was told that audio books would be furnished through the state education dept. I'm 23 and feel like your husband but am going to try.