My son was falsely arrested for failure to register as a sex offender. 2 days later he received a letter & call fron the our Sherriff's Dept. stating that the whole thing was a mistake. The charge was dropped. He was told that he never had to register in the first place. What are his legal options ?

Towards what end? You want to open a can of worms by suing the sheriff's dept? Mistakes happen. If the sheriff had even the slightest justification for arresting him then he has no case. I would strongly suggest that he drop this. Maybe ask for an apology, but that's about it.

I would just drop it, first he could not have been arrested without a warrant, so a judge had to review evidence and sign a warrant.