My son pleaded guilty to a felony, which a judge reduced to a misdemeanor after he completed probation. Is he still considered a felon?

Your son is not a felon.  His prior felony was reduced to a Misdemeanor.  He should always report that he is not a felon.     Your son should inquire about an expungement if 5 years have passed.    When potential employers run a background check, will it show the felony plea or will it show the misdemeanor?   when non lawyers read a background report or court docket they will think it is a felony but upon more discerned reading it will state that it has been reduced to a Misdemeanor.  Always a problem with lay people reading background reports and jumping to conclusions.  You have to explain this sometimes.  Or get it expunged so most people will not see it.