Fifth-grade boys NEVER sing in public. At least not for people they don't know very well. Something happens over that previous summer, and those fun-loving, unselfconscious fourth-graders becomes cool, not-putting-myself-out-there fifth-graders. We know this.
So, as a visiting artist working in schools, my typical tack is to warm them up with plenty of humor, try to rivet them with something authentic and compelling, and THEN ask them to sing. They WILL try at that point, but not very committedly.
Then, I ask the girls to sing, and sure enough, they raise the roof a bit.
There's really only one thing left to do at that point: you simply guilt the boys into not getting outdone by the girls. It works every time. (It also works with adult males in reserved Yorkshire England, and tenth-graders everywhere.)
Recently though, I visited a school where the fifth-grade boys ACTUALLY SING. Pre-guilt! I had no idea what to do at that point!