I suspect Goodwill don't sell undies - I know our local op-shop won't sell second-hand underwear.
And Karen, I'd also noticed his similarity to your S2BX before.
I would be tempted to buy him one last set of underwear - buy seven. Then use fabric paint (or permanent marker) to label them Sunday, Monday, Tuesday, Wednesday...
Or if he's a real grot, buy 12 sets of undies and label them January, February, March, April...
Again I'm reminded of one of my favourite authors, Terry Pratchett and one of his characters, Cohen the Barbarian (aka Ghenghiz Cohen). He's very elderly (because he's been so good at surviving every battle), has a haemorrhoid ring on his saddle and wears leather loincloths and nothing else. When a pretty young lady politely suggested he needed to change his loincloth, he replied with surprise, "Why? Good leather don't rot for years!"
Marg