Urgent!!! Is my puppy diesel SICK?

I have a pitbull who is about 8 months old and he is sneezing and coughing quite a bit he has done this for as long as i can remember, we got him at about 8 weeks , he has
   
all his shots and up to date on heart worm medication i do tend to be a little worry wart i mean he binks funny i wanna bring him to the dr so i just dont know what to do PLEASE HELP ME? we brought him to the vet and they said nothing seems wrong but he never coughs when we are there they have give him some meds that worked for a minute but it comes back but mind you he eats and drinks and goes to the
   
bathroom fine and he has TONS of energy i mean nothing is abnormal about him at all except the cough and sneeze
   

Dogs suffer from seasonal allergies as we do, so it could be that the pollen is getting to him.
   
If the vet examined him and thinks he's fine, then he's fine, particularly if he's energetic, eating, drinking, peeing and pooping all right.
   
If you want you can buy a cool mist vaporizer and run it near your dog's sleep area at night.
   
This will help clear his bronchial passages if he is getting some mucus and will not harm him if he is not.
   
Do not run the vaporizer in the same room as a television with a DVD player, a CD player, computer, etc. as over time the moisture can build up in electronic equipment and foul it up.

thank you very much