According to an IRS FAQ regarding CARES Act questions, employers who received PPP funds can defer Social Security tax payments until they are notified of the PPP forgiveness (See FAQ #4 in the link below). Once the PPP loan forgiveness is confirmed, the PPP borrower can no longer defer any future Social Security tax payments, but what they deferred before the forgiveness date is allowed to be deferred until the date shown in FAQ #7 in the link below.
To read the IRS FAQ, click here.