What does the invitation say? If it specifies you by name only, then +1 or "and guest" (usually with a line where you write the name of your +1) then you don't have to ask. A plus one is intended to be for a (ideally long-term) partner, but it is technically for anyone - within reason: not somebody who is coming solely to enjoy the open bar, somebody the bride hates, somebody who is specifically NOT invited so you bring them anyway, et cetera.
If they are expecting your BF as your +1, it would be polite to let them know "the BF couldn't make it but so-and-so is attending on his behalf." But you were afforded a +1 so you wouldn't have to attend alone.
But if the invitation says both of your names specifically, it is against wedding etiquette to try and "transfer" the invitation so to speak.