[KWH]

Lets dissect this a little.
You say he takes a step for and reset.
Did he reset to a NEW position? (THIS IS A REQUIRMENT FOR A LEGAL SHIFT!)
1) If your answer is NO you have a false start!
2) If your answer is yes AND he remained motionless for a count or longer you may have had a legal shift, however is it more likely that he missed the snap count. If so, false start!
3) If your answer is YES, and he did not remain motionless for one second prior to the snap, you most likely do not have illegal motion rather you have a false start.

All things considered, IF YOU HAVE A GUT FEELING that the only reason he moved is because he missed the snap count you have a false start!
Nothing good can heppen by allowing this play to continue so shut it down and enforce a false start penalty.
When the whole world knows he missed the snap count, this is the right thing to do!