Can I sue the state if my son was 16 when sentenced to an adult prison?

It is legal for a judge to sentence a juvenile to adult prison if the judge makes certain findings. The state did not put him in prison, the judge did. The judge is absolutely immune from a lawsuit about decisions he made as judge. The Department of Corrections employees have a complete defense to a lawsuit about his being in adult prison, that is, that they were obeying the order of a judge. If he was in solitary confinement for a year, that possibly could form the basis of a lawsuit, if a court were to find this action to be unreasonable. You would need to determine which people arranged for the solitary confinement in order to find out whom to sue. They will defend against the lawsuit with reports or testimony about his bad behavior or psychological evaluations while locked up, which they will claim made the solitary confinement necessary.