Can I use 999-88-9999 as a social security number if my baby still hasn't receive his real social security number yet?

No.  The child must have either a valid Social Security number or an ITIN issued by the IRS for you to be able to claim them as a dependent on your tax return.

You must have a valid SSN for the child on an "on time" return in order to get any tax credits.  If the child is eligible for an SSN, wait for it to arrive, even if you file late.  Don't file without the child and don't use any other number for the child.  You can file an "on time" tax return up until October 15, 2020, IF you request the automatic extension by midnight on July 15, 2020.