maybe hes having parasites infection, I had one betta that had similar infections to the betta in ur photo. I treated it with parasite treatment and ot got better. Maybe u should be aware :-
It could also be a fluke, or (less likely) even ich (which looks like a white spot, which I suppose is similar to a pimple!), which is also treated with an anti-parasite medication. I'm guessing treating it as a parasite would be a good way to go, and then see if it gets better.
Furan-2 or Jungle Parasite Clear are good medications. If it is Hexamita (the cause of "hole in the head") then Furan-2 is the way to go, since it is readily absorbed into the body, and hex is an internal parasite