Reluctancy to Eat

Question
Golden Retriever; 3 yrs; intact male.  We took him as a rescue late December.  He's a lovely dog and has no problems except that he is underweight and in spite of quantity of food given doesn't gain significantly (he has a light bone structure) and is certainly faintly disinterseted in food. He's been tested for any fundamental problems but that was negative.  Last couple of days he has eaten very little and won't eat the dry pelleted food we give our other dogs. It could be as simple as a stomach upset (loose stools)which if it continues I'll check with the vet after the bank holiday.  Our concern is that being underweight anyway we need to get something into him.  Chicken and rice are on the menu today, gave him a little yogurt this morning.  Any suggestions of nutritious appetite tempters; both in the short term and after, once whatever has upset him has passed through?

Answer
It is highly unusual for a healthy dog not to eat all it needs.  If his ribs aren't showing, he doesn't need to eat more.  Your dog definitely should be narrower at the waist than the hips and chest. You should be able to easily feel the ribs, but not see them. Each dog is different. Standard recommendations are a good place to start, but each dog must have its food and exercise adjusted to its individual needs. Here is a link to a good illustrated guide, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx

If he has remained so thin his ribs show after 5 months, perhaps you need an opinion from a second vet.  

The chicken and rice is good for settling an upset digestive system.  The yogurt may help too.