define unmovable.  Does it feel like part of the skull?  Every cat is
different, and maybe his head is just built differently.  However if it is
squishy at all, or feels warm, it could be an abscess - from a bite wound.
(Could be any number  of other things too)

If he is still acting fine, and has input and output (eating and what not)
and has no other changes, you could wait.  However, keep an extremely close
eye on him.  My kitty twee got an abscess on her neck.  I noticed that it
looked a little odd, and she seemed "a little off" but my husband - who
loves her VERY much - kept telling me I was silly. (I have a tendency to be
paranoid)   But she had a slight fever, and a very deep abscess.
Fortunately I caught it in time that antibiotics worked.

It feels like bone and does not move at all. He is acting perfectly normal.
I wonder if he has had it all along but it is getting bigger as he does.
Thanks for the reply. We will watch. I joke that I pay at least a couple
months of my vets mortgage. Seems like I am there monthly for yearly
check-ups!