L.
Hey there. I just wanted to say I know what you're going through!! My first was 8 months, but my second grew teeth two days after he turned 4 months!
One thing we did, was gently press him in towards the breast, and he will let go. That's one option. But, another thing we did, is remove him from my breast, and gently (but firmly) press down (with your finger) on those teeth that are emerging. Say something like (as you're pushing down) "No Bite! This HURTS Mommy!" It doesn't have to be those words, and don't yell at him, but he needs to understand (as much as possible at this stage) that it hurts...he WILL get that. After you do that, you can put him down for a few minutes as well.This shows him that you are serious, and that his actions have consequences. I didn't have to always put him down, as mine caught on quick, thankfully!
If I think of anything else, I will be sure to let you know! I wish you the best, and hope he quickly learns!! :)