Other than letting him know that I love him and am there for him, he has to choose when to take care of himself. You are absolutely correct that you can not badger or parent him into taking care of himself. All that you can do is tell him that seeing him not take care of himself makes you angry and sad since it seems to mean that he does not care enough about himself to want to do everything that he can to insure that the two of you will be able to share a long and mutually healthy life together. The whys are enormously complex. Adhering to the rigorous medication schedule is arduous and a constant reminder that one has a life thretening illness that needs to be treated.
Many people report taking a drug holiday.
Another possibility is that he is suffering from either a low grade depresion or a more serious depression and his not taking care of himself may be one symptom of a more general lethargy or malaise. This transition from someone with a lethal illness to being a person with a treatable but now only potentially life threatening condition is not a simple one to navigate. Any combination of these dynamics or other ones that are unique for him can all be contributing to why he has suddenly stopped taking care of himself. Try to notice if there are other areas of his life where he is not taking care of himself like paying his bills, housework, pet care, etc. If it is a more generalized condition than just the meds, he might benefit from professional counseling.