Do you think the pain and lump could be from a milk pimple or blocked duct, early mastitis, thrush etc?

Usually pain from damage, thrush etc will slowly disappear after attachment and unless there is a problem with attachment the rest of the feed should be pain free.

Usually the only way to stop little bad habits is to stop the feed and put them down, letting them know it hurt.

Maybe have a lc look at his attachment and see if it has changed? Did he get any new teeth recently, it can upset them a bit?