Bump on Dog's Leg?

I have a Pit/Lab mix. I got him last May and we love him completely. A few months ago he got a mole on his back leg, and over time, it's gotten bigger, being the size of a dime now. It is completely round and raised above the skin. It started out a reddish color and felt almost like a ball of cartilage or something. When you move it, the whole thing moves. He's started licking it a lot, and now it's blotchy and white and pink. I think he started licking it because it had a crusty top. I scratched it a little, and brought it to his attention. I don't think he noticed it before. Now I think he has licked the skin clean off of it. Thank you all.