Over at the Gentalman's Guide to Black Tie (I think that's what the site is called) they say there should be four buttons on the jacket cuff, unless they have changed that.

I did not go to a prom, so I have no idea what goes on there.

I would want a silk, real silk, pocket square, and I would prefer a full size one, so I could fold it the way I prefer, rather than a swatch on a card.

And, of course, real gentalmen tie their own bow ties.

Hope he has fun.