You have two choices, it´s up to you and how you want to handle this. Allow the child to continue with the bananas and yoghurt. And allow chips ONLY as a special treat, like dessert. Bananas and yoghurt are very nutritious, so don´t worry about im not getting a balanced diet
http://www.whfoods.com/genpage.php?tname=foodspice&dbid=124
And bananas are a meal-in-one, with so many nutrients, why bother if he only eats that?
http://www.whfoods.com/genpage.php?tname=nutrientprofile&dbid=92
When sticking to the bananas and yoghurt diet, then leave him be, to eat as he wishes, while the rest of the people at the table eat and enjoy their meat and veggies. Sometime or another, he will realise that there is more to life than simply bananas and youghurt. And, really don´t bother about the chips, there is very little nutritional value in them, so use only as a treat, once a day.
OR
you can go the hard line, and serve only what is on the table, and watch with a sad heart how he eats nothing, all the while you are worrying that he is hungry and not getting anything to eat. Do not give in to the banana and yoghurt pleas. As others have said, he may start eating other food when he realises that he is going to get nothing else, and no one is reacting to his demands for yoghurt and bananas.
If it were my child, as a mom, I would have had my worries about this diet. But, now I am older, I would definitely let the child exist on his bananas and yoghurt diet, because I now know how healthy that combination is. Just see that you buy natural yoghurt with live cultures (probiotic) with no added sugar.
I think, that your grandchild will, on his own, start asking to taste other foods. Just don´t push it. When he sees that it is no big deal only eating bananas and yoghurt, he may find this becoming very boring.
Bananas and yoghurt are covering his basic nutritional needs.
Good luck