Does my puppy have a rash or allergies?

I have a 3 month old pit bull puppy and he is black with a white underbelly. He is constantly nipping, scratching and biting himself. He does not have fleas, and is on Frontline. Underneath his armpits, legs, and some parts of his belly are scratches and red marks from his incessant biting. I understand some dogs are allergic to corn and many puppy foods contain it. He also eats grass any time he lays down outside or is near it. He has never thrown up or been sick and does not eat the dirt. I want to break this habit because some grass is treated with pesticides and i do not want him thinking it is ok to eat it everywhere we go.

If those areas he's biting at are red I would lean toward an allergy problem.
Look at the inside of his ears, between the pads of his paws, his abdomen and any other exposed skin areas to see if they're red. If you see this chances are good it's allergies.
Food isn't the only cause. They're could be underlying issues. It could be the frontline, the grass, lots of different things.

The link below provides a good look at the different causes of allergies. Might help you determine if that's what you're dealing with and how to go about treating the problem.

It could be a food allergy, but some puppies develop puppy pyoderma or "puppy acne". My dog had it when he was a puppy and the vet thought it was a food allergy - so we switched his food to crappy Science Diet Z/D (which is NOT meant for puppies) and put him on all kinds of steroids and pills... only to have it go away on it's on by 7 months. Now he is one year old and eats a good food, but he can now have wheat without any symptoms (which is what they thought he was allergic to). Anyway, it's probably best to but your pup on a good quality food, but also ask your vet about the puppy acne. Good luck!