Washboard might help but the greater issue is the germs in this sort of laundry.
Remember this sort of waste has E. Coli and other bad things. So underwear really should be washed in very hot water and/or dried in a high-temp dryer.
Maybe if you used anti-bacterial soap in conjunction with the washboard that would be OK, but if there's a lot of this type of washing you're doing, you could be creating a health risk from touching "clean" clothes that aren't.
And I for one would make him wash his own clothes.