Hi Jennie! Fatness is result of it's diet and a danger to it's liver and heart. As long as frog has fat reserves and is healthy; not eating is OK. Also, it's winter, so that slows their metabolism down.
If frog is accustomed to large prey, you could try to feed him a "frog sausage." You make those with human sausage casings and can put inside crickets/dubias/night crawlers (for movement) and even prepared foods (Pacman, Repashy's Meat Pie). Make one about frog size and wiggle it in frogs face. If it's hungry it should strike it

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