"Guy" Bridesmaid?

Have the girls wear a black dress, to match the black suit. Or have the person who is wearing the suit wear a vest and tie that is the same colour as the bridesmaid dress.

They make tuxedo jacket dresses - not sure how much or where you could find them, but they could be an alternative.

I think Attentea hit the nail on the head

My sister has a lot of friends in the trans community, and even though you are totally used to calling him your sister, if he identifies as a man, no matter what stage of transitioning or not, I would suggest you use male pronouns and refer to him as your brother unless he specifically said not to. It can be really confusing at first! http://www.glaad.org/reference/transgender

With the trans issues to the side, bridesmen and groomsladies are not that uncommon today. If he wore a suit/tux, I'm sure that would look great, and I don't think the bridemaids would have to match too closely.

I am having a bridesman in my wedding. He will just wear what the groomsmen are wearing, and stand with my bridesmaids on my side- easy as that

I have a man of honor and he is wearing what the groomsmen are wearing. Easy-peesy!

It might be tough to accept, but your sibling is not a woman anymore, or at least he's transitioning and does not want to wear feminine clothing. I recommend asking him what he'd like to wear. He may prefer to dress in whatever the groomsmen are wearing.

You have a guy bridesmaid, not a "guy" bridesmaid.

I also have a man of honor and he will be wearing what the groomsmen wear, but instead of a tie, he's wearing a bow tie :o)

I agree with Brittany's points, although of course it all depends on your sister/brother's wishes. My best friend's sister just identified with female rather than male and she hasn't had any operations but does have hormone therapy and we refer to this person as a her now, because it's what she wants.

I like all of the attire options that others have suggested already

It takes a lot to get used to, but letting him stand proud beside you-- that's awesome!

Just get him to wear what the groomsmen are wearing and stand beside you, or the same suit they're wearing with a tie the colour of your BM dresses. This is fairly common, not to worry.

To answer your question, they dont have to. If hes the only guy up there, people will understand why hes not in a matching dress...

And the term is bridesman,

I would have him wear what the groomsmen are wearing with a tie and/or vest matching the bridesmaids.

My friend had a man of honor and he wore a tux with a tie that matched the ladies.

I'd recommend asking him what would be the most comfortable for him.