I actually had the same situation; I just graduated from college; over two summers, I worked for the same company. The first summer I worked through a temp agency, the second summer was directly for the company. On my resume, it just says, "Company XYZ, 6/98-8/98, 6/99-8/99".
Basically, even though your husband technically was employed by the temp agency 9since they're the ones signing the checks), he was still doing the same job, and the fact that he was working through the temp agency for part of that period of time isn't really as relevant as the work that he did there. And if a prospective employer were to contact anyone to verify employment, it would more likely be the actual company he worked for rather thasn the temp agency.
And I ended up getting a job with the resume set up the way it is, so I guess I was doing something right....
Hope this helps.
-Steve