This fellow is a friend of Jackson's sister. So basically, we all know him. Or something. He is an excellent arranger of a capella AND a master of the green screen as well (which is how he manages to replicate himself.) He doesn't do the beatboxing- that's his friend.
Thanks for the tip, Jackson!
By the way, the world's most (possibly only) famous a capella group, Rockapella, will be in town next Wed, Dec. 14 at symphony hall. Tickets are very expensive- $40- but I'm sure it'll be awesome... (Hat tip to Linda T for the heads up!)