I worked for agency as a cna in that vicinity , most of the agency gigs for cnas are for LTC but there are some acute care gigs out there as well (if you have experience) basically you give them your availability and they call you with a job and where its at. you can always say 'yes or no' the thing to be careful about is that once you go to that place with the agency you are locked in a 6 to year long contract where you can't work for that facility unless they buy out your contract. A lot of people don't understand that coming in. good luck
To get a per diem job at the hospital would probably be more ideal but he may not be able to pick up as many hours as he would with agency. it just depends what your looking for.