"The fish is his dinner."
Translation:De vis is zijn avondeten.
simply because it's "de vis". I'm afraid in most cases you need to learn this by heart.
Animals though are normally "de", since they are either male or female opposed to the neutral "het". De vis, de vogel, de beer, de olifant, de tijger, etc
It's only "het", when you make it into a small animal: het visje, het vogeltje, het beertje, etc
I grew up in the South-eastern United States. Not sure if this is a regional thing or a my mom thing but Dinner is the biggest meal of the day. Growing up, most days dinner was supper, but on Sundays we'd start dinner as soon as we got home from church and eat sometime midday with a lighter supper in the evening.