I had this thought too! A bit of a weird way to begin a romantic relationship with your reincarnated mentor(?) and the king he only remembers as a child
Right now we are at a point where only Karas knows it's Abel so he's being possesive and protective. Abel has not even a slight idea that Karas is the little boy he took care of so everything right now is his own innocent feelings.
In the novel Abel does struggle with his feelings towards Karas even before finding out but he does address that he is conflicted due to Karas now being an adult and having adult desires towards him alongside the type of adult Karas has become.
The romancey stuff will end soon-ish to continue the main storyline with Tryude and the black poison so Abel is genuinely taking what I'd consider a respectable amount of time before giving his all
It's hard to imagine that Abel will fall in love with Karas considering he views him as a child. Hope he falls in love with Karas as just captain commander and finds he's his majesty later.