If he is suppose to weigh around 10lbs then he should only need about 2 cans of Fancy Feast a day since they are 3oz each, I know my 10 pounders eat a 5.5oz can of Friskies per day, but that is once he is regulated, while he is still unregulated he will need to eat more because his body can't use the food he is eating properly. Both of my diabetics before they were regulated were eating 2-3 cans a day of Friskies, even little Autumn who barely weighed 5lbs when I adopted her even though her ideal weight was 14lbs (she is a big framed girl). However, now that she is regulated she only eats 1 can a day and is maintaining her weight just fine. With cats like humans if you need them to gain weight you feed more, if you need them to lose weight you feed them less.
Mel, Maxwell, Autumn & The Fur Gang