Bump on bf’s ass

My bf gets a bump right at the top of his ass cheeks, right between the cheeks- all the time. He’s a driver and says he gets it from the bumpy truck. I looked at it and it has pus inside it. Looks bigger than a pimple. What could this actually be? He’ll get one then it eventually goes away then some time later he’s get one again. He says it’s very painful and he refuses to see a doctor.