Cut raw into small pieces and eat it slowly. As a spice or something like that - it adds flavor when mixed with other foods.

For example, you can put butter or lard on bread, spread those small pieces of onion on it, add salt. It is actually good.

You can also cut it into thick ovals and bake. Third option is to caramelize it. But, these two are time consuming.

A white onion can be eaten raw thinly sliced. Perhaps a dash of vinegar but that might be too fancy for his tastes.

Besides baking as the other comment said, you can also caramelise onions on a frying pan with a tiny bit of oil, they're quite nice that way.

If you like stronger flavours, you can add your favourite salad dressing to a chopped or sliced raw onion (I usually add extra virgin olive oil, red wine vinegar and oregano).

You can bake onions. Quite nice.