What about- the kitchen is closed?
If they dont want to eat a lot at one meal, why not split up dinner? It could be that he isn’t hungry when dinner is served, or he cant eat so much then so he is becomes hungry later. So maybe have part a before the rest of the family has dinner and eat part b with the family. Or tell him if he wants to eat later, he can have a fruit ect.
Eating occasional cereal is ok, but it should not become a habit. He should be trained to eat well when he is younger so that when he is older, he will make better choices.