Overweight 2 year old

My son is overweight, he’s always been overweight. He was losing weight when he started walking but his grandparents feed him junk when they have him for a weekend visit. I asked them not to feed him junk food, just fruit for snacks, water & his meals. She said she doesn’t feed him junk food but she lied. When I picked him up, or video call he’s munching on a big bag of chips. Yesterday when I picked him up, candy was everywhere. An empty candy bag, half eaten candies on the coffee table, a bag of gummy worms by the TV. I’m disappointed. lol.

He finally stopped drinking milk from a bottle so hoping that helps some. How can I get him to lose some weight in a healthy way?