I'd buy several pair. Maybe start a go fund me page if you have to. Otherwise just rinse them in fresh clean water at the end of every day. If you have a little oxyclean, or other sanitizer, then add it, but rinse twice with the last being just water. Let dry in the best breeze you can, but if they aren't completely dry the next start time, why care, they'll be sweaty soon anyway.
Underwear will just increase your chance of getting sores from wrinkles and abrasion. Underwear won't make anything any better. It's the microbes, fungus and stuff that will grow in the moist fabric that will bother your skin. They need to be removed daily so their population is kept to a minimum.