As others have said, he is a grown man and people do have to take responsibility for their own health. He's no age, he could have years to live if he looks after himself, it's a shame. You can't be responsible and nor can your husband, if your FIL is of sound mind. By all means go see the GP for a chat to convey your side of the story, but he may not be able to discuss it with you, but at least you will have said your bit and asked him to note it.
Unfortunately some people are the authors of their own misfortune - do you think he's afraid, do you think that asking the GP to make a (rare) home visit and talk to him straight might help?