Puppies sometimes don't eat at first, but he should be eating by now. Often not eating relates to a health problem or over feeding. If you didn't get him to the vet this week, I would next week.
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
Many puppies come from the breeder overweight. Unless he is too thin, I I would put down the dish with what the dog should eat, and give it 15 minutes to eat. Then take it up. Do not give it anything to eat until its next scheduled meal. In a few days, it should be eating what it needs. Continue to check its ribs and adjust the food as needed. This is not easy. I had a Shepherd go 3 days on a few nibbles. I was a wreck, but she was fine. It is almost unknown for a healthy dog not to eat what it needs. Unfortunately, in too many cases, it is less than the package says, and less than the owner thinks the dog should have. Many dogs are quite good at holding out for tastier chow. Like kids, sometimes it calls for tough love.