labrador wont gain weight

Question
hi I have a 5yr old lab i feed him 600grms everynight and dry biscuits in the mornings he just dosn't seem to be gaining any weight? I always worm him and frontline him he's very active and was the biggest puppy when i got him he has mated a few times and since then i noticed his weight deterating rapidly the more i feed him the more he shits so i just don't knows anymore he doesnt look healthy please suggest something for him

Answer
Do his ribs show? If not, he may be closer to a healthy weight than the dogs you are comparing him too.  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

What does the vet say?  If his ribs show no matter how much you feed, most likely there is a medical problem.  

I am not a big advocate of grain free foods, but the more meat and less grain a food has, the less you need to feed to maintain a good weight.  Most labs will do fine on less than 400 grams of such common brands as Iams, Pro Plan. or Pedigree.  If you are feeding a food with corn as its first ingredient, you might move up to one with a meat meal as the first ingredient.  Those with an agenda have filled the internet with utter nonsense about canine nutrition.  One thing that is true is that meat based foods produce smaller, firmer stools than grain based ones.  For a more scientific view of dog food than most internet sites, see http://skeptvet.com/Blog/2009/07/pet-food-nutrition-myths/