"Which" is correct, although it is a very formal sentence, and we wouldn �t say it this way.
In this sentence, "who" would be a pronoun, which would substitute for the word "man", so we would have �
He bumped into the last man in the queue, who fell down.
Since "man" is still there, we need an adjective, in this case "which"
He bumped into the last man in the queue, which unlucky man fell down.
Again, this is grammatically correct, but very stiff and formal sounding.
Bruce