Having worked with people who are dealing with addicts in their life, I would advise you to put your focus on what you need to be eating, on food preparation for your own health and feed him what he asks for.

He is not asking for anything illegal or immoral, it is cheap, easy to prepare and he needs to gain weight and is willing to eat it.

It is your job to control what you eat, not what he eats, regardless of how much you love him, no matter how right you think you are.

If he is dealing with significant medical issues, he may be dying and it may be too late to heal it through good diet.  If that is the case do you want the last weeks and months you spend with him to be a battleground?

If he is not dying, then this diet will not kill him overnight either, he may have time to come around to eating a better diet by watching the benefits to you over time.  But you can't argue, plead or reason an addict into changing and the effort just wastes your inner peace and progress in your own life and way of eating.

Kathleen