I recommend that you come up with a non-verbal signal to let him know he needs to quickly finish up what he is saying. Discuss it beforehand so you both can agree on what the signal is and what it means. Make it something that looks normal for two people in a relationship, perhaps putting your hand on his forearm or something like that. Nothing too large or obvious that would bring attention to it. This could be used when a conversation about a topic is going on too long, or when it is time to finish up talking so the party can end. It might take some getting used to, and it is dependent on your BF being willing to be advised in this way.
I give my husband a "look" that signals that I have about 15-20 minutes of sociability left in me and it's time to get ready to leave. He then starts the wrap-up, letting people know we have to start heading home.