he's a Norwegian Forest cat, so is quite huge but he also is rather... rotund! he's about 5 now (he came from a rescue centre) and I just found a photo of when we got him 3 years ago. blimey.. he's fat now!
he really doesn't seem to eat much, quantity-wise, and he can only eat dried food (Iams) because wet food upsets his (fat) tummy.
he's also a lazy so and so, and sleeps belly up for most of the day.
Science Diet for neutered cats has less calories but apparently still feels filling. Lucy the pussy seems to like it. Although - I am sensing that Colin may be a bloke and it may not have a good effect on him.
I initially got a supply from the vet free, that was when my cat had just been neutered and they were testing out the food. Last time I got it in the pet store (Pets at Home).
You can get Science Diet anywhere, its only the Hills Prescription Diet that has to be got from the vets (but they won't tell you that, they will just charge you a lot more than buying online or at a pet supermarket). I just ordered a 15kg of Purina cat food online, free delivery and £18 cheaper than my local pet supermarket.
there is obviously a limit to the minimum amount of food Colin needs to keep life and limb together - obviously you've sussed that so why not try and find something to stimulate him to play. There are lots of weird and wonderful cat toys around like feathers on a pole, wind up mice etc - think the key is shaking him out of his lethergy. also, if he was in a rescue centre for a while there is just the chance he's gotten out of the habit of playing. Presumeably he's had blood tests etc to check he doesn't have an underlying medical condition?
There is Hills diet food from the vet which you can get if the vet thinks it appropriate. I know yours is a big breed but how much does he weigh? I have three ordinary moggies who are fit well and active and all weigh more than 5kg, they are big boys but not overweight.
Sorry forgot to add - it's my understaning that this breed is very adventurous and playfull so perhaps there is an underlying issue. Either that or you just have the only couch potato known to the breed!