Roberts himself changed the mandate into a tax in the majority opinion. If he planned to kill it as unconstitutional then, why kick it down the road? Why not just leave it as a mandate, which was already unconstitutional?
Start at line 24, bottom of page 16.
http://www.supremecourt.gov/oral_arguments/argument_transcripts/11-398-Monday.pdf
I don't think he could due to the Anti-Injunction Act. For a tax to be challenged it must first be assessed, which none yet have. I did read some commentary a while back from someone who said this might be something Roberts would do.