Thanks for all the replies. It's interesting to see the range of opinions clustering around 'pick your battles and buy more t-shirts' and 'give him limited choice and tough it out'. I can see the merits of both, but at the moment I am inclining towards the 'limited choice & tough it out' approach. One practical difficulty is that I can't very easily, I've now realised, get replicas of the 2 t-shirts. Basically it's a superhero obsession. One is Batman and was bought for him by relatives in another country! The other is a Superman t-shirt that I bought as the last one in his size on sale nearby - though I could do some internet searching and probably locate more. Also, I could look for more superhero-themed t-shirts which would probably be accepted!
I do feel a bit despairing, though, at the thought of spending money needlessly on clothes when he already has plenty of good clothes, in designs that till a week ago he loved and was happy to wear. Fortunately, spending money on clothes isn't going to leave me short of food (for some people the cost would really be an issue with that approach) but, even so, it's money that could go towards paying off my credit card, house repairs and plenty of other important things. I do get the 'pick your battles' mindset on this and that the money spent might well be worth it to avoid the upset. I am actually a pretty soft touch, for anyone thinking I am a hardline disciplinarian! But I do worry that it will be the thin end of the wedge. He has started recently to be much more demanding about asking for new toys, clothes etc to be bought for him - I am a bit concerned about seeming to pander to that too much.
On choice - just to be clear, I'd always have allowed him a choice of t-shirts from the ones he owns and has clean - I'm not so daft as to make an issue of me having to overrule him on the design of his t-shirt every day. But I am doubtful about buying more straight away (will not rule this out altogether yet) and don't want to consign myself to washing t-shirts every night so the treasured ones are always clean! When I've told him before that they're in the wash, that also prompts a meltdown.
Things were a bit better today. My DH told DS yesterday that the 2 acceptable t-shirts were going to be weekend t-shirts so that he could wear them when we were all together and out having fun as a family. He accepted that at the time, asked for his Batman t-shirt and cried this morning when I said it wasn't the weekend. I did get out 2 other t-shirts for him and gave him the choice between them - the same for socks and pants - and he did then choose and seemed OK with it. We've been having the same issue with pyjamas but I have let him just have his favourites for several nights now. I will have a chance to wash and dry those in a day when I work from home on Fri so it's not quite so bad.
Once again, thanks for the views. I am glad it's not just my DS as well as feeling like I have some options to try!