> he looks a lot smaller than the others
Jeangenie has given you good advice. There is quite a variation within this breed, even within the KC breed standard. Your dogue may well be OK for his frame. I don't think it matters as long as he's healthy.
For what it's worth our dogues are raw fed and eat 2 meals a day. The biggest who weighs around 60 kilos eats approx 1.5 kilos at his evening meal, and a bit less in the morning. The bitches who each weigh around 50 kilos have about 1 kilo in the evening and a bit less in the morning. It's not an exact science. Three of them I'd say are just the right weight for their frames, the fourth is an older neutered male who is inclined to gain weight if we don't watch it, so he is given less poor devil!
All our dogues eat a variety of raw meats during the week, tripe chunks, beef chunks, heart chunks, lamb, with turkey neck or chicken wings for bone content. They also eat eggs several times a week and tinned tuna, with a daily spoonful each of spinach liqidised with pear or carrot. They have a spoonful of oil (fish, sunflower and wheatgerm) each evening. If you do a search on here you will find lots of discussion about feeding, both raw and complete.
You could try adding any of the raw meats to his diet in place of the tinned food and see how that goes. And a spoonful of oil will be good for his skin and coat. But if he looks well covered as he is and he's in good health, I wouldn't worry that he's on the small side.