Ahha I might be able to help since I just had a test over carbs, protein and fats today.

Carbs and protein both equal the same amount of energy. Fat equals 2.25 more energy than carbs or protein do. Excess fat in the diet gets stored in the muscle and liver and excess carbs gets stored in muscle whereas excess protein just gets flushed out of the system. I would obviously stick to the diet your vet prescribed, but typically you'd want a diet higher in fiber to help weight loss. At least, I hope I got all that right, otherwise I probably didn't do so hot on my test

But not all fat is bad. It is actually an essential nutrient for the body so don't always assume since theres more fat that it means it's unhealthier. Obviously there are exceptions to everything but typically it's more beneficial than not

Good luck getting him back down to a healthy weight!!!