When my son got sentenced the papers said 24 months at 100% so he went in on Jan. 4th so does that mean he will get out on Jan. 4th 2020?

Was this sentence related to a probation/parole violation? Normally, a 100% sentence is related to that - where the first act of leniency by the courts was not heeded, there is no second chance at freedom. If this is the Judgment and Commitment paper signed by the judge on the original case, then he will be entitled to the "automatic 15% good time credit" allotted to all inmates when they begin their bid. The inmates can only lose this by not following the rules.