The guy likely has 0 dating experience,
and on top of having a resting bitch face/poker face, likely doesn't have a high social skills (he said so himself that he has an "unpersonable personality").
He's also seems to be the serious type and the reason he doesn't treat her as nicely as he should is because he's holding back due to their promise to their parents; they're not husband and wife until she graduated.
So of course like the usual serious character, he takes that promise seriously and is keeping it. But due to his bad social skills and his face, he comes off as uncaring.
I think japanese people call this type of guy as 'herbivore men'. Usually this type of character gets paired with a 'carnivore girl' but instead here he's paired with an innocent and naive girl, so it will be interesting to see both of them learning how relationship works together.
And if you're still complaining, you can see this work isn't tagged with Ecchi or Seinen. I'd recommend looking elsewhere instead.