Statements made by a litigant in the context of the litigation are "privileged" communications and can not be used against the litigant as the basis for a defamation lawsuit or other type of lawsuit. You would also subject yourself to a potential Anti-SLAPP Motion to dismiss your complaint an if won, then you would have exposure to pay for the attorney fees of the other party in defending the lawsuit you filed. In sum, based on the facts you have given, you do not have a viable action against the person telling the lies and you could end up with a judgment for substantial attorney fees against you if you do file.
Please note that this answer does not constitute legal advice, and should not be relied on, as each situation is fact specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship.
No, likely not. But you can continue to be a good influence in this little child's life, by (1) giving him plenty of love, and (2) NEVER trashing or undermining his mother. You, and your relationship with the child, will very likely suffer.
Professional Rules of Ethics require me to advise you that this is an offer of possible representation.