Parrots are like kids. They want to do what the grown ups do. If you're eating mac and cheese, potatoes with butter, chips, chicken McNugets then Fred is just copying - he's doing what you've taught him to do from social conditioning.
My suggestion? Eat some fruit infront of him and act like it's the best thing you've ever eaten. Then when he's getting curious give him a tiny bit (obviously making sure you don't give him a bit that could have your saliva on.) If he nibbles it - lots of positive reinforcement and praise. Introduce lots of things this way and see how you get on. Bella wouldn't eat grapes until I did this with her.
If you're feeding a mix, put some veg through a food processor and then mix it in with the mix, that way a bit of goodness will get in.
You can make veg more palatable by adding a smidge of coconut oil or palm nut oil and mixing it in. I've found that my birds love oven cooked butternut squash - cooked in a little coconut oil. I mash it and feed it warm. It's a great comfort food.