I was going to tell you how lipomas (fatty tumors) are common but now this does not sound like a lipoma. The only thing that you can really do it keep it clean and protected. I would wash it well with peroxide and keep it dry. If he's licking it or is bothered by it, you can slip a t-shirt over his head to prevent it.
Then see your vet on Monday to find out what's going on.
Let me know if you still have questions.