Beagle Pup is having feeding issues

Question
I just got a beagle puppy a few weeks ago. I was told he had already gotten his 1st set of shots around 6 weeks. I am taking him to get his next set in a few days. He is around 8 weeks now. He was eating just fine when we got him 3 weeks ago, now is wont eat much and his tummy is a little sunken in. His stools are usually quite runny, but some seem to be normal. He is mostly in a good mood and playful, but sometimes he just seems to walk around. I feed him kibble and minis because he is small. This is my first puppy so i'm not really sure what ne needs to be eating. My daughters let him get into their food sometimes, mostly dry cereal. I'm worried about him, i want him to be healthy. My husband plans to use him for hunting when he is older...so we want him to be a healthy little man. Any advice? I'm willing to suplement his food some if thats what is needed. Thank you for your time.

Answer
With modern dog chow, supplements usually do more harm than good.  I wouldn't do anything until you get him into the vet.  It could be worms, an infection, parasites, or other physical problem.  Most dogs will thrive on most chows.  It doesn't sound to me like an over feeding problem as is common.  

You should be feeding a puppy chow now. Switching to an adult chow for the larger breeds at 4 months, slows growth and helps develop sturdier joints.

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