Is he lethargic.. or is he his usual self...
In summers dogs should be kept in a cool place in the plains as they can get a heat stroke just like us, so checking his temperature is a good idea and if so he may have to be put on a drip etc depending on how serious he is. The vet would advise. It is very difficult without seeing a dog and its surroundings to tell what can be the matter. But some other things are as follows:
Labs are greedy dogs and will eat anything + he is still a very young dog.....he may have eaten something which he is not supposed to. Is he kept open in the house and was he left unattended before this happened. Due to boredom dogs can get destructive and again eat something which they should not have eaten.
My advise is that ask your vet to feel his stomach and see if he can feel anything abnormal or pain and ask him to do a ultrasound/ sonography, if not then take an X ray of the stomach to see if my case the dog has eaten something which is stuck in his stomach. If so there are two options then give a injection to make the dog vomit and if that does not help then it will have to be operated and removed. But please go to a very good vet.
Thinking of it now please also check your dog's body for any injuries/ bites and or smelling on his paws and legs, if your dog goes in the garden or park areas. It can also be a snake bite, since nothing has happened so far we can hope it was not a poisonous one, and due to the shock the dog is not eating. A similar thing had once happened to my boxer, but I got him anti venom within a few hours to be on the safe side.
Please do not worry, but I am giving you possibilities as at times vets forget to check for other things and the pets suffer. All will be ok, just get him properly checked by a vet and be calm.