well, it's called a power struggle. he is growing up and exhibiting his frustration to get attention and his way.
It's totally normal for a toddler to do this. It's also totally normal to scream when mommy is away. Why? because he can. And if he hasn't had anyone BUT YOU for his life - with only small increments of daddy - it's a huge change...
Your fiance did NOTHING to distract him from the fact that you were gone, right? or he said he tried to distract him? but it didn't work, right?
your finance says his son doesn't like him. Okay. then your fiance is putting off vibes and your son can feel and react to those vibes. It could your finace is feeling uncomfortable being "daddy all the time" instead of part time...more responsibility, etc.
Now that you are living together...I'm sure there are additional struggles..finances, expectations, getting used to each others habits....all that jazz....I take it you moved into his parents house or got an apartment together instead of buying a home you couldn't afford?
If you are concerned about the head banging - talk to your son's pediatrician. Find other ways to help him cope with the stress of change and being told NO. Oliver is testing his boundaries. Oliver is seeing what he can and cannot get away with. This is TOTALLY normal. Now it's a matter of how you handle it. You "SAY" you ignore it - but I'm sure your body language and words to your finace are a totally different story - Oliver knows it upsets you - so he does it.
As for your surgery on Tuesday? Looks like daddy is going to have to deal and be a daddy. This means taking control and setting boundaries. He will have to learn how to play with his son and take care of him on his own. You are obviously worried about it - and since you are concerned/worried, Oliver is picking up on that as well...it's amazing what kids pick up and work with....