Tuxes-question and opinion

My question is- We only have 2 men in BP. FBIL and FH's best friend in Florida. FBIL is a GM and FL man is BM. FBIL asked if he could wear a black suit instead of tux; as an aside I only have two women, my cousin is MOH and FSIL is Bridesmaid. FBIL and FSIL will walk down together with their two children. Do you think a black suit with a nice tie is ok?
We might also have BM wear a black suit as well.

ETA- My girls are both wearing different dresses, bought them on their own.

I think that Black Suits look just as nice as a Tux.. We went to a black tie affair and my husband wore his black suit and no one could tell the difference.
Question - how do you feel about it, does it really matter?
Also were you planning on the guys wearing bow ties or regular ties? What I would do is probably have both of them wear the same tie.

I guess it depends. Ive seen some black suits look just like tuxes and some that dont. Do you trust this guys fashion sense to be sure its black (sometimes its hard to tell it black unless it up against something that is truly black) and appropriate for something formal?

Keep in mind that since he is the only other man, his suit will stand out even more if it varies greatly from the tux.