Of course it's "objectively" against what you consider to be his self-interest and happiness, and yours. Now all you have to do is show that your definitions of self-interest and happiness are themselves objective, and not simply your personal, subjective preferences for long life and thinness. After all, this is supposed to be completely objective, right?
It is logical, objective, and (I would have thought) obvious.
It's none of the above, it's simply you defining your values as rational, and other values as irrational. Why is long life better than a short life? For that matter, while you're pondering that one, why not objectively prove that it's better to be alive at all rather than dead?
Have fun ;)
Eating cake breakfast, lunch and dinner is "objectively" against his self-interest. Because eating that way will have effects that will be "objectively" bad -- poor health, weight, under-nourishment, etc.
It is logical, objective, and (I would have thought) obvious.
If you're just arguing to argue, enjoy yourself tho.