VETSCOPE

hello. so my dog is an almost 2 years old, chihuahua mixed breed. he doesn't have worms or any health problems at least according to 4 vets i took him to. the problem is that he is very skinny (4kg). he doesn't eat a lot but on most days he eats all of his 3 meals (i give him cheese, a mix of chicken thighs + pumpkin + rice or boiled potato, sometimes i give him human food because he is a picky eater and gets fed up with the same taste) the problem is that he never gains weight and i'm worried about him. i keep thinking maybe his body is not able to digest nutrients? for the past week or so i started giving him supplements for dogs (calcium in the morning then a multivitamin + iron later on) but i see no progress and i'm very worried. please help me