You've cast yourself as the martyr.
You're not overweight because of your family. They don't hold you down and force the biscuits in.
What's their weight like?
So I have to on my days off think of his breakfast, lunch, dinner and snacks
No. You don't. Stop it. He is not a toddler. He knows where the kitchen is.
He will always say no when I offer something
So respect that, LISTEN to him.
but I feel sorry for him so I make something
Even though he's said no? That isn't about him, is it.
And he will gobble it up within seconds at his desk which makes me feel bad for him!
This is not healthy. He isn't a baby bird needing food crammed down his beak.
It’s a vicious cycle, I’m angry I have to keep thinking of feeding him but then feel guilty if I don’t.
No, you don't have to keep thinking of feeding him.
He's a grown man. Not a cluster-feeding baby, a toddler or a chick!
And he's not a boiler, either. Nobody needs constant stoking.
Just stop it.
When not at work, and he's WFH, have other things to do. Go out, go for a walk, go swimming, the park, taking exercise that will immediately benefit you.
If you must, leave breakfast cereals, the makings of sandwiches in the kitchen. If he's that weaponised-incompetent-enabled, teach him to microwave a bowl of soup and pull a baguette apart.