A 12 year old is not eating for the sake of nutrition. FTR, there are very few cereals as far as I know that have no sugar. They include Shredded Wheat, Puffed Wheat, and Puffed Rice. As far as I know all others have sugar in them. Cheerios is only LOW in sugar, though very nutritious.
I am not so into letting a child not eat his regular prepared dinner, but I would provide a variety of healthy choices. And even if he could help with the cooking, though many kids incluiding boys enjoy that, the family can sit down on Sunday evening and make a menu plan for the week together. Occasional sugary treats can be sprinkled in among the “good” stuff.