Weight problems

Cimbi is thin. It is not conspicuous becaue of his fur, but when you touch him all the ribs can be felt and also the backbone....
This is a new problem to me as all my dogs and cats were, well, let's face it, overweight.
Cimbi eats 3 times a day, a small portion of breakfast and supper and a generous portion of lunch. He eats only homemade food, soup with vegetables and plenty of meat,  rice or noodles, often oatmeal.
We consulted our vet already a few months ago. He was not worried as Cimbi is healthy and in excellent condition but as I insisted, he gave some vitamins to improve his appetite.
A few weeks ago we returned to the vet to weigh him, the scale showed 56 kilos again. It may sound a lot but it is not for a dog of his size, He should be minimum 60 kilos and I am at my wit's end ...