Meeting with MD - how to delay acceptance

So, I'm apparently going to receive an offer from a mid-level IB boutique and meeting with MD is scheduled next week. However, I already interviewed with a EB (think of Roth/Laz/Ev) and looking forward to interview with two BBs (GS/MS/JPM/BAML/Citi). All positions are supposed to begin on Jan 2017. Thus, I would like to take some time before accepting mid-level boutique offer in order to see if something happens with other banks. Which is the best way to handle this situation and how should I deal with the incoming meeting?

Any help would be much appreciated