large boots stores also are quite good.
i found some tight-fitting mens hipster type underpants could hold a pad securely, and using standard underwear may be more acceptable to the user; depends what they are used to, but its worth a try. the square=shaped, with legs, having some lycra in the fabric.
not triangular, y fronts.
i would avoid referring to continence, far less incontinence often at night with someone having dementia. if it can be presented as helping with the laundry, for prostrate problems, or not having to get up so often at night. some way that sounds like an useful item rather than part of an unravelling.
it can be tricky. perhaps the person is unco-operative as they find it affronts their dignity, or more fundamentally, the fear of dementia. hence why i suggest talking around it.
no good trying to batter them with realities / logic. can be too harsh and ineffective, causes more anxiety, stress, resistence.