Just a thought - have you tried pulverising some of the dry food and sprinkle that on top of his wet food?
I feed raw myself, but sometimes my cat isn't as interested (especially if it's straight from the fridge, I think it being cold makes it smell less) and a sprinkle of the Thrive chicken treats on top always gets him going
Might be worth a try? As the others have said, the best thing you can do that will help your cat lose weight is to remove the dry food and all carbohydrates from his diet.
Good luck