From what I read, he gave up the SSL cert by printing out a hard copy in a tiny font, and when he was ordered to provide a digital copy, he shut down the service.
> At approximately 1:30 p.m. CDT on August 2, 2013, Mr. Levison gave the F.B.I. a printout of what he represented to be the encryption keys needed to operate the pen register. This printout, in what appears to be four-point type, consists of eleven pages of largely illegible characters.
And:
> On August 8th, rather than turning over the master key, Levison shut down Lavabit.
