Fatso (1980) on Netflix

Dominick DiNapoli has always been a big kid who loved eating. It was his favorite thing. Then his cousin dies from health complications due to a lack of exercise and improper diet. Antoinette, Dominick's sister, makes him promise to see a diet doctor and lose some weight. This is very hard for Dominick, but he tries. He also finds motivation when he meets Lydia, and he discovers a love that is more intense than his love of food. He spends so much time kissing and walking around with Lydia that he no longer eats as many unhealthy things, and he loses weight without even trying.