Stu:Post-Completion OPT and resident alien status

Good day:

We have a Resident alien employee who is Stu:Post-Completion OPT.  Although he is set up as a resident alien, I do not see where he can be set up with an F1 status. He is exempt for Medicare and SS but those liabilities are still being deducted with the Resident Alien status.  What am I missing?