As with so many things in life, there is a balance to be struck here.
The op’s dh has a right to dress how he wants most of the time.
But equally, as he is part of a marriage in which they sometimes presumably go out for the evening together, I don’t think that op would be unreasonable to want her dh to dress up occasionally, simply because he knows it is important to her, even if it is not important to him.
Aren’t little compromises like this part of a successful partnership? For example, I would be happy if I never ate steak or red meat again, but I know my dh loves it so I buy and cook it for him occasionally and we both enjoy it together. And then I do enjoy it because it makes me happy when my dh is happy.
Op you hint that your dh may be holding out on this precisely because he knows it is something you enjoy, and if that is the case, then this is not really about clothes is it but your wider relationship?
In order to find that out though, it might be simpler just to say to your dh, look we are going out on Friday evening next month for this special event, and I understand you are not bothered by clothes, but it would really give me great pleasure if you dressed up a bit because you look so handsome in that blue suit or whatever. Would that be ok with you?
If you want to you could also add a phrase such as for you, foreplay starts long before dinner.
(I don’t care if that seems horribly sexist btw. Women have been expected to make aˋ effort with their appearance for decades so this addresses the balance somewhat.)
Just tell him straight op, if you haven’t done so already, sometimes men need it spelling out. And then if he still refuses, or doesn’t at least give you a reasonable very considered explanation as to why not, that you can understand and accept, then at least you know you have a greater problem on your hands.