Black Lab skin problems

[Yayo2012]

My 2 year old Black Lab has been having many problems. He has been on antibiotics for the last 3 weeks for a rash on his chin. I am also pretty sure that he has an ear infection that I have been treating with drops that I already had from the vet. For the past couple of months he has been chewing on his butt. At first we thought that it was just out of boredom, but now we cannot get him to stop chewing. He is constantly chewing on his butt. We now noticed that there is discharge also. Since he is already on antibiotics, I do not believe that it is an infection. BUt, I do not know what else it could be?
The vet does not believe that anything is wrong with him. But, I have had this vet push off other issues my dog has had, that turn into bigger issues later on. I am not sure if I need to have him seen by a different vet or if this is normal behavior.
Thank you!

[Margot49]

OK.  
Any flea issues?
You using any type of topical treatment for fleas and/or ticks?
You might try switching foods to a grain free with Omega fats in it.

[Yayo2012]

I feed him Iams dry dog food. He has metal bowls, he has used the same kind since he was a pup. And we do not treat the lawn at all either.

[Margot49]

Few simple questions
What are you feeding?
Do you use plastic bowls for the dog?
Any fleas, even just one bite can set off a reaction in a sensitive dog.
Have you treated your lawn with any chemicals?
He may be starting a staph infection on the rump.
Even though he is on an antibiotic, if he has an infection, it may not be the right one.
If I have to question a Vet I go to a second time, I get another opinion.
He may or may not need a skin scraping done.