How does my son get a divorce while in prison

You should go talk to some family law attorneys, report your results back to him, and coordinate with him by power of attorney or other means to contract with a lawyer to represent him. The lawyer will then represent him in a divorce. Of great importance to the attorney-client relationship is whether any parent-child relationship is involved, the size of their marital estate, etc. Yes, he can get a divorce from in prison, and it probably can be filed in that county where he is presently residing or in a county in which he claims to have permanent residence. The lawyer eventually may want to meet with him at his unit either to decide to take the case and/or to determine fee retainer, case details, etc.

The very first step will be figuring out jurisdiction. There are special allowances in the statutes that address incarcerated parties.