I totally understand. With my own experiences, I do not like it when people ask me about stuff that I, myself, know I shouldn't be doing. As far as what I'm gathering, it's the same deal here. When you bring up "addiction" and he goes into full defensive mode, that's because he knows its true, I dont really know how to scientifically back any of this up. But trust me on this one, you are not overreacting and in fact you might actually be "under"reacting. (Again I dont know full details, etc).

Stay strong and just try to be supportive without "stepping on his toes". And that is HARD to do! Also, make sure you are thinking about yourself, too. If its hard to care for your S.O. without them raising their defenses, maybe there's more to this than just Gambling.