Hi Susan and welcome :)
Regarding your question about jury service, I don't know much about the exemptions as I've never been called up and hope I aren't! However, if your husband is on good CPAP therapy and approved by the DVLA to drive (if he's a driver) then I can't imagine it would be classed as an exemption. However, if he isn't on treatment yet, it may be something that can be looked at if he's suffering extreme daytime fatigue.
Sorry to hear that Susan. If he's still so tired it would be worth asking for help with the clinic in case his pressures need adjusting (assuming he uses his CPAP for all sleeps of course, as that would cause tiredness). If his therapy is ok then it could be something else causing the symptoms so a good idea to get the GP to run blood tests.