it sounds like an abcess. Which I can imagine can be caused by a tick bite. When he's had them before do they just go away on their own? Abcesses are caused by dirt or other things getting into the skin and causing an infection and they are usually filled with pus and the like. The tick could have been the irritator. If it keeps getting bigger, you might want to have the vet come out and drain it, which you can do yourself, but if you have never done it before, best to let the experts show you first. But I can tell you how anyways.
When you touch it, push in different spots to see if you can find a soft spot. This is where the skin would be the thinnest, and where it would eventually pop out on its own. Its called the path of least resistance lol. That is where you would take a sharp needle or even a scapel and make a small hole. Then it should drain, maybe having you gently push around it to encourage it. I would clean the area with some rubbing alcohol first, and after, to make sure its somewhat sterile. And maybe give him a little bute for pain, but just relieving the pressure is going to cause a great bit of relief. Just beware, abcesses can grow huge in small amounts of time, so it is best to have the vet come out and make sure it is one, or if it is something else. I've had vast experience with abcesses since my horse gets hoof ones quite frequently in the winter. And since you said it is rock hard, that makes me think it is one even more.
Best of luck to you and I hope your boy gets better :)