Apologies all for not coming back to this sooner - it's been a busy week for me. Some of these responses...! 
God I needed a laugh, thanks for that.
I'm finding this hard to believe
1) - that anyone would ever do this
2) - that OP would question whether this was right or wrong
StaggerLeee I'm afraid this very much did really happen. I pinched myself to check if I was dreaming and everything. Nope, still here, very much really happening, this is my real life, take this cup of suffering from me etc. I wasn't posting my original question to ask whether I was in the wrong so much as checking that I hadn't gone completely mental as DP made it sound like I was the unreasonable one!
Addressing the common questions/expanding further:
Weirdly, no, he is not usually smelly. Which is why I didn't notice this sooner, I suppose. And I guess I'd just ~assumed~ he was washing things more often, as he puts on the occasional load of laundry or hand-washes something in the bathroom. I've recently quit my job to write and stay at home with DS (10m) so I have slightly more time on my hands now, but before this I was working full-time so I wasn't paying attention to how much of his stuff was making its way through the wash!
Apparently his pants rotation works like this: wear a pair, 'air' a pair (i.e. leave them ON THE FLOORDROBE), rinse a pair in the shower (why not just wash them properly...), throw them in the wash when they need washing (I assume this means when they smell). This is slightly better than what I was imagining but not by much. Isn't this is the kind of thing you do when you're staying at a hostel, not living in your house??
Yes, the sock situation is equally vile. But on this front he doesn't even have an excuse as he's got fucking loads of socks.
However pollymere your point about the older generation washing things less often is worth considering - he is almost fifty and grew up in Nigeria. He was also living alone for several years prior to moving in with me and I think growing up his mum did everything for him, so I suppose grotty bachelor habits die hard. (Not that I think that's an excuse but I'm ~trying~ to be understanding)
I was trying to avoid infantilising him or being that person who nags all the time but that obviously that hasn't worked and I don't want to live in a pig-sty, so... [defeated shrug]
Well, lesson learned. The pack I bought was a multipack of 12, so he's got plenty of pants between washes. In future all pants and socks will be whisked away on sight. I guess now I know I can keep on top of it.