You definitely need an attorney. Child abuse and false imprisonment are both felonies and punishable with prison. Obviously an issue in the case will be whether your stepson lied or not, so you will need to get evidence showing that he did in fact lie. Your attorney can conduct depositions where he will interview your stepson. But anyone charged with a crime NEEDS an attorney.
Do not post anything online where someone could try to give it to the police or use it against you in court.
Feel free to email me if you have any questions.