To wonder why he seems incapable of saying thank you?

[SpeedbirdSquawker]

I took my mum and her stepdad out for lunch today. My mum always says thank you but he never does. Any time I have treated them to a meal or night or two away he has never said thank you. I don't do it for the gratitude for them but a simple thanks wouldn't go amiss. Am I expecting too much?

YANBU and this would genuinely put me off doing it again.

No you're not expecting too much. He's a rude arse.