My three year old was never picky but after a weekend with Granny, all he wants is chicken nuggets and mac and cheese

When I say my son wasn't picky, he wasn't. This kid used to eat chia seeds, spinach, every fruit, every meat, and every veggie. This kid was a dream to feed. Whatever we eat, he used to eat.

My mother-in-law used to make him other food when we visited, but we reminded her several times that we don't make him "kid food", just kid sized portions of what we're eating. She eventually stopped and for the last while made him kid portions. Do we eat chicken nuggets, fast food, and mac n' cheese? Of course! But he only eats that when we're eating it, which is not often.

My husband and I went to a wedding on Saturday, but we dropped off our 3.5 year old with my husband's mom, our son's "Granny". We sent him with fruit and veggies, and reminded her that she didn't have to make special food with him because he'd eat what they're eating. Well, Friday is fish and chip night. Apparently our son only had fries and "hated" the fish and peas (which he's been there for fish and chips and loves everything). Then Saturday and Sunday he didn't want what they ate for breakfast so they gave him Frootloops because asked for them (he's never had Frootloops before so I have no idea where that came from). Apparently, he only ate fries, McD's chicken nuggets in plum sauce, chocolate milk, juice boxes, and ice cream all weekend.

This is incredibly unusual for him. If he doesn't want to eat, he doesn't want to eat anything. He rarely refuses food and demands something else. We only eat fast food if we're on the road, like driving to my parents. We hardly ever bring fast food home.

It's now Tuesday and he has been calling every food "yucky", "gross", and "Mommy's poo food" and making a face and pushing it away. He's demanding "Granny's yummy food" and "nuggies".

I am about to lose my mind. I have never dealt with this before. This is the same kid who, last week, wanted chia seeds and mashed blueberries (we call it frog's eggs and smashed beetles) for breakfast, but this week wants Frootloops. He also refuses to drink anything other than chocolate milk or juice. We don't have chocolate milk or juice in our house and this kid screams bloody murder.

Please tell me that this is a phase he'll grow out of. Advice would be greatly appreciated!!

And my husband and I talked to his mom. She denies calling our food gross, yucky, or poo food, but this is too much of a coincidence. It's just very frustrating but that's a whole other issue we're dealing with.