My dad wears diapers and won't wear pants. It upsets his grandchildren to see him without clothes on.
Does your dad have difficulty zipping or buttoning the pants? Try sweatpants or something easy on/easy off. (adaptive clothing)
Do the pants fit him (i.e. do they droop and require a belt)? Try suspenders or easier w/elastic wasteline.
Lay out 2 pairs of pants and letting him choose which one to wear.
FYI there are sites online that sell "adaptive clothing" for people with dementia/ALZ. Good luck!