J.R.
I almost weaned my first daughter when she was 5 months old because she was biting! We got through it though, and I nursed her for 14 months. I never used formula for 4 kids--you can do this!
Many babies do bite at the end of a nursing session--and just as you are seeing--it is a game to them. He really doesn't understand that he is hurting you. It isn't until babies are about 14-15 months old that they start to understand that what you want might be different from what they want. If it is fun for them, they think it is fun for everyone--and seeing mommy yelp is obviously fun for your son.
So you need to be proactive. Watch him closely as he nurses, and when he starts to slow down, take him off for a burp. Then move right to playing with him. If he's still hungry he'll let you know, but if he's just about full, he should be fine.
With my DD it was her top teeth that were rubbing, so be prepared for that. She wasn't actively biting. I needed to work with some experienced moms (love the La Leche League group in Lehigh County!) to learn how to position her on the breast so that she wouldn't scrape.
Good luck!