After the arrest by the SS last year, his right to own a firearm was suspended and his guns confiscated and given to his dad. The dad gave them back to him. The dad's likely going to be in some trouble over this.
I doubt the Dad will suffer any legal consequences in giving his son with mental illness back his guns that the son used to kill four persons at the Waffle House. We'll probably learn that the son had expressed to the family his continuing interest in murdering others and they did nothing.
Gun laws in Tennessee have few restrictions. The state doesn't require a permit to purchase or possess a long gun or handgun, is a "shall-issue" state, concealed carry reciprocity with thirty-five other states, NFA weapons are not restricted, background checks on private sales are not required, nor any restriction on assault weapons, magazine capacity, or owners to be licensed and the state laws preempts all local laws. All certifications are required to be completed by the state in fifteen days.
Firearm disability is limited to those who have been adjudicated as a "mental defective", judicially committed to or hospitalized in a mental institution, those who have had a court appoint a conservator for the applicant by reason of mental illness, developmental disability or other mental incapacity, and has not, within seven years from the date of application, been found by a court to pose an immediate substantial likelihood of serious harm because of the mental illness. The son would suffer a firearm disability due to any felony conviction. A 2009 Tennessee law requires submission of mental health records to NICS. The state’s circuit courts, criminal courts, general sessions courts, county/probate courts and chancery courts that have ordered a commitment to a mental institution or that have adjudicated a person as mentally defective required to report this fact to NICS and the Tennessee Department of Safety.
In 2015, Tennessee also enacted a law establishing procedures for an individual prohibited from possessing firearms under federal law for mental health reasons to petition for relief from the federal prohibition. A person who is subject to federal law’s mental health-related firearm prohibition because the person has been adjudicated as a mental defective or committed to a mental institution may petition the court that entered the commitment or adjudication order once three years have elapsed from the person’s date of release from commitment or the date of the adjudication order, whichever is later.
Mental health professionals are required to report to police patients they judge could seriously harm or kill "reasonably identifiable victims." So the Dad of the murderer might only suffer consequences if he was the court-appointed conservator of his son who was adjudicated with mental illness. Otherwise, he is not required by Tennessee to report any worsening mental illness and/or dangerous behavior of his son in possession of an assault-style weapon to the police. The son may have stopped any follow-up visits to mental health professionals and stopped any prescribed medications. Unleess the father was a court-appointed conservator, he would not be required to report and mental health professionals would be unaware of these changes.
The NRA has fought in state courts laws that restrict gun possession by those who have been involuntarily commtted to mental institutions, arguing violation of their Second Amendment rights and violation of rights of Due process under the Fourteenth Amendment.