A bread machine can make the "make our own bread...toast" less laborious. (Recipes printed in bread machines' instruction booklets tend toward the less-healthy, but with some experimenting...)

Commercial breakfast cereals vary, though the least-healthy are probably the most tuned to appeal to kids. Home-cooked cereal can be far better. And easy to mix nuts, frozen berries, etc. into.

If a bit of dressing up (squirt of mustard, bit of mayo, etc.) makes hard-boiled eggs acceptable to him, those can be made once a week and kept in the fridge.

An old friend perfected a fairly-healthy, filling stew recipe (some meat, mostly veggies, etc.) that he could make up in large batches and keep in the fridge. As a teenager, his son consumed an enormous quantity of that over time.