Usually he won't be able to tell there is mustard in it. My DH is not too keen on mustard either but when it's in salads he does not notice. Just put in a tiny bit or use lemon or vinegar as well to give it a bit of a kick.
Otherwise it's just pototoes, mayo, salt, pepper and egg or pickle if it takes your fancy.
you can do all sorts of things like adding bacon, caraway seeds, celery salt.
Spanish style uses a nice olive oil mayo with lightly cooked peas, carrots and even tuna. They call it Russian potato salad for some reason.
My mom makes a really good one using: Potatoes, onion, red and green peppers,and boiled egg ---all finely chopped. Also add in some tomato -squeezed well to rid excess moisture. It's seasoned with mayo, salt, pepper and a tinge of cider vinegar. It's wonderful! The potatoes almost all apart to make a mix between chunks and mash.
The type of potato also matters and you want to use one that is good for boiling. :-)