It's very solid, feels like as if it's made of bone. I am thinking he may of knocked himself pretty hard maybe and it's occurred due to it, but I have no idea what it could be exactly. The owner of the place said he has seen something similar on a horse once before, and he said it turned into a cyst and they could then easily drain it. But I honestly have not come accross something like this before. I have tried googling things on it, but can't seem to find anything. He is only 7 years of age, if that may have to do with anything :P I have felt it, and its very solid and if I squeeze it relatively hard he does have a reaction to it. I honestly have no idea what it may be.
I took some pictures, and they are just below. But advice would be very greatly appreciated.