@clawcliphurts
It's all about tolerance and 'horses for courses', isn't it? Your parents aren't 'wrong', his mum isn't 'wrong', they're just different.
The problem is that whereas you are accepting of the differences in your parents and his mum and just get on with things, he is intolerant and unaccepting of your parents 'ways'. And he's verbal and insulting about it to boot! Personally, I'd get the ick with that. I could deal with him saying "I'm not going to your parents anymore", but not with to him giving me hell about the way they are. Nor would I tell my parents not to make a cup of tea in my kitchen. It's OUR house, I'm just as entitled to treat my guests as I wish to as he is. And if he doesn't like it, he can absent himself.
I have no idea of why he's so intolerant and frankly, I wouldn't really care. What I would care deeply about is his insulting words and behavior (especially if it's obvious to your DC), and his rigidity. The former is so disrespectful to you (and your parents if he doesn't trouble to hide it), the latter would worry me as far as how it might affect our children as they get older. Is he going to be this rigid about school, hobbies, friends, etc? If he is so rigid about being invited, sitting up to the table for everything, and guests not using the bathroom that's going to have a negative affect on your DC's ability to have friends over and may inhibit their ability to make friends if they don't feel comfortable having them over because Dad insists everything be 'just so'. It may also drive them out of the house and into their friend's homes. I certainly wouldn't have felt comfortable if I felt that every time I went to a friend's house I felt like I was going to a state dinner at Buckingham Palace! OK, that's hyperbole, but you get my point.
And as an older person myself, I have seen how when people age many of their characteristics, good and bad, often become exaggerated because we tend to get get set in our ways. Is he going to start demanding that you emulate his mother? Is he going to start having tantrums or become hugely angry if things aren't done 'his way'?
You've got a bigger problem that needing to put doilies on plates.