Yep that a male, and if they got staff tha cant sex guppies then i would not buy from them as this is the most basic thing some one in a shop would know.
Yep thats a male! In the pic you posted at least. Can you show us an actual pic of the fish in question?
Right now I would think you should quarantine him and treat the ICH!
Raise the temperature before treating, you raise the temp to 28C then wait until the white spots fall of the fish - usually within 48 hours - (so you cant see them at all) then start treatment.
All whitespot treatments do the job as they are mainly all good... i always use interpet.