My gelding had this for quite some time, it popped on its own though and drained but was on the bone in the same area. It just kept getting bigger and it was hard but if you are worried about it, get a vet out and see if they can drain it. Everyone seems to jump to strangles right off the bat. It could be something as simple as a calcium build up. Get the vet out.
I truly hope for you sake it isn't a dental infection, I just went thru this with my mare. She had a lump on her right jaw bone that started to drain, well I was almost sure it was Strangles, however I was very stumped on how because she had her vaccinations about 3 months prior to that. However it look no different than strangles...It was a dental infection and shot of general anesthetic and a huge bill cured it! ;) Sugery!