Limping lab puppy 16 weeks old

Question
Hi I have a fox red lab puppy now 16 1/2 weeks old. His face is still small but his body seems to have grown quite big and his paws are huge. He has been limping for the last week and has been lethargic for the last two days. We live in a very hot country where it is currently 47 degrees celsius. He had diarrhea a day ago but his stools seem to be normal now. He is eating normally but his activity level seems to have been reduced noticeably. He has been given all his shots and seems to be healthy otherwise. Please advise.

Answer
The best thing is for him to be seen by a vet.  

Both overweight and excessively rapid growth can lead to limping or even bone damage.  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