Maybe try peppermint or ginger tea in the morning to help with nausea (it can be hot or cold, whichever he prefers). He does need to eat. Between the not eating in the morning and the focus on weight loss, I would guess he may not be eating enough, which is going to contribute to him feeling tired and run down.
Potatoes are fine. Is it possible to eat too many? Sure. But if what gets him to clean up his diet and eat less chips and ice cream is potatoes, it seems like a good trade off, and a stepping stone to more veggies later, when he's ready.
Whole30 can still work for him, but he's really going to have to put some effort into it. Maybe have him find Whole30 recipes he'd be willing to try, or go to the store and pick out a vegetable he thinks might be interesting to try. You can cook all the healthy food in the world, but if he won't eat it, there's not really anything you can do to force him to, that's something he has to decide for himself. Sensory issues certainly make it harder, but they can be dealt with, and if they're what's holding him back from having a healthy diet, it might be worth having him talk to a doctor or therapist about them, but even with professional help, he'll have to work on it, so it really needs to be something he wants to do.