This business of, when you're invited for a meal, taking something along with you seems to be very much a regional/cultural thing.
When I was young and first married, it was very much the accepted practice to arrive with a bottle of wine, or some after-dinner mints, flowers or a pot plant. Occasionally, we'd be asked over to friends and asked to take something specific - a dessert or maybe some cheese. When I hosted, I would do the same. But we were all young and hard-up and it was a great way of socialising with a small group with nobody feeling put-upon.
Until relatively recently, I'd always continued this habit of making a contribution, however small because that was the custom.
But then I moved to a different part of the country and met my now SO. When old friends wanted to meet me and asked us over to eat, I naturally made sure I had some wine, chocolates or whatever to take with. Both SO and his friends and family thought this behaviour very strange.
"Scribbles, they've asked us to dinner. I promise they will have all the food and drink anyone could possibly want."
And so they do. After a few episodes of polite thanks and bewildered looks, I've stopped doing it. When in Rome, etc, etc!
So I wonder if the OP's mother's partner simply comes from a different background where this isn't 'done' so it's never crossed his mind and mother, as close family, sees no reason to behave like a guest.
Either way, I had the unhappy feeling that the OP doesn't much like mother's SO but, unless she wants to fall out with her mother, then she must accept they are an item and he will accompany her. Unless he has some seriously antisocial habits, how difficult can it be?
