its a pretty big bump to be a rubbing calus. but i'm not saying that isnt what it is.
could be that he ramed something pretty hard and damaged his lip. if its getting bigger, i would add some salt to your tank as well as Malafix.
That will help it heal quicker and prevent any bacterial infections. Or if one is starting, it will help get rid of it quicker.
Dose with the malafix for a week, but keep using the salt until it is healed. And of course W/C's. It doesnt look too serious. Just an injury/calus that is trying to heal.