Mollie again this is talking in very general terms.
I would broach the subject by asking if it is ok to talk about it.
Than say the effect whatever it is has on you. How it makes you feel - worried about his health, what the condition will lead to, eg type 2 diabetes, cardiovascular disease, premature death etc. Stress the effect the behaviour has on you and above all avoid criticising or blaming him.
Then ask what, if anything, he can do to change.
Take it from there and support him in whatever he is willing to do, whether it's seek help from the doctor about dieting, or go to a gym, or read about low-carb diets on the net (plenty of good advice on Gransnet).
That's the theory - easier said than done I know, but very often people don't realise the impact their behaviour has on others around them and when they do, they start to see things quite differently.