It depends on what you want and how he asks (and why). Is he a vain narcissist? Or is it for health reasons (example: family history of diabetes, hypertension, high cholesterol, etc). If we know we are prone to some terrible ailment due to simply eating too much and choosing not to be fit/healthy, then why should anyone support us in our bad choices in the long run? Don't you want a partner that encourages healthy choices?

I left someone a long time ago because when we met she expressed she wanted to get back to her high school weight, but then years later she was consistently gaining weight year after year, and rather than talk about it she was argumentative, defensive, and it drove us apart. I would go away for a week and come back and she would be 15 pounds heavier from bingeing. It was too much for me. When I asked her to seek help she punched me and told me to leave her alone. So I did. And now she and her new partner are each over 300lb and need so much medical help they probably won't live to see 60 years old. At least she found the partner she wanted and seems to be happy!