i would always recommend a referral course where you start in a dive center at home for the theory, video watching, written test and everything you don't want to spoil beactime with. the class commences on makunudu with the required dives and you would get your certificate.
the PADI open water course would be appropriate .
as for your husband being diabetic , the only person to answer your question is his doctor .
my hunch is that he will be fine, given some ground rules and qualify for the diver's health certificate. but anyone not familiar with your husbands specific health situation will not be able to ethically advice you here.