Ahoy, Colleen!
How exciting that you are in the planning stages of a Disney Cruise! For the past couple of years, Disney Cruise Line has celebrated "Halloween on the High Seas" from mid-September thru the end of October and had
"Very MerryTime Cruises" from mid-November through early January.
The official holiday dates for 2016 have not been released, but be sure to watch the Disney Cruise Line website for the announcement. I am sure that your brother will have an amazing time, no matter the decorations onboard!
For a lower cruise rate, you are right to be searching in the late November/early December time. Since children tend to be in school during these times, the cruise fares are lower than over the school holiday breaks.
As for Disney trading pins, the Crew Members who work in the shops onboard all have trading lanyards or hip lanyards hanging from their belts with pins on them to trade. You can also ask to see the "pin book" in each shop to trade there as well. There might be an "Officer Pin Trading" night as well, be sure to check the Daily Navigator to see when this event might be happening.
I hope you and your brother have an amazing cruise! Please come back and visit the Disney Parks Moms Panel with more questions as you are planning your fantastic vacation.
Happy Cruising, Tracy V.