WOW, I don’t think you should comment, just do as he asks. You can smile of course, wish him a good day, or just say “of course, my pleasure”. If you make a big deal it will back fire. If it is his decision it might last. If you make seem like he is doing it to please you, or he is back on YOUR track, it might back fire. As I said before it might be an influence of one of his Rebbeim or someone at the school. It might be something the entire group is trying together, so just watch and see without letting your heart catch in your throat. Step back and just be a bystander while he finds his footing.
As far as the simcha is concerned don’t discuss it with him until and day or two before. You will be surprised what he might decide on his own. A few days before the simcha you might ask him if he needs his suit pressed or altered for the simcha or if he needs a new shirt. Are you planning on having the men wear matching ties, or coordinating ties? If so, why not ask him for help choosing the ties for the men.
If the men are not matching in any way and he doesn’t want to wear a white shirt try to compromise, see if he will agree to a light blue or light grey shirt with a nice tie. It is NOT worth fighting over. The fact that he is there and participating in the simcha is the important thing. What he wears in NOT that important do don’t focus on that. You can tell him that it is a “formal” affair so dressing down would not be appropriate.