You may be thinking, “But this guy never dresses up.” Fair enough, but that doesn’t mean that you can’t point him in that direction. Also, if you do give him a certificate for formal wear, it doesn’t mean that he has to purchase a full tuxedo.
If he’s more of a casual guy, he can use the gift to buy a nice tux shirt and cufflinks and studs. Or he could decide to go with a dinner jacket and tux pants. With those, he might wear a white shirt with an open collar.
Of course, he could decide to use your gift to purchase a full tuxedo. Although it’s true that men’s formal wear can get expensive, it’s also true that you can purchase a tuxedo package for as little as $135! With those packages, he will get everything he needs to create a full formal wear ensemble (except for the black shoes).
By the way, please don’t try to pick the tux out for him. Could you imagine if he tried to choose your next formal gown or dress? That’s not a good idea. You want to let him make decisions regarding what he likes, what he looks good in and what he wants to wear.
This does not mean that you can’t be involved in the process. But don’t push. You can make suggestions and help him make decisions if he wants such help. Encourage him to have fun with the various choices he has at hand.