With a twenty-year ten-suspended sentence, your son is serving the active portion and will become eligible for parole consideration based on the rules of the state he is in. Most states require an inmate to serve a minimum percentage of the active sentence before their first parole hearing, typically somewhere between a third and half of the active time depending on the jurisdiction and the offense. At three plus years in, he may be approaching that window depending on how his state calculates eligibility. His case manager at the facility can give you the specific date his file goes before the board.
On what you can do to help, the honest answer is that the parole board hearing is almost entirely about him and what he has done with his time inside. There are no attorneys in the room, no advocates making arguments on his behalf. It is the board reviewing his institutional record and assessing whether he is ready for release.
What sets the table for a successful hearing is built long before he walks into that room. A clean disciplinary record, completion of any programming the facility recommended, participation in education or vocational training, and demonstrated remorse and self-awareness are what parole boards respond to. None of that can be manufactured at the last minute.
What you can do from the outside is write a letter of support that speaks to his character, his family ties, and the stability he is returning to. Some boards consider these letters and they can reinforce the case he is making in the room. Beyond that, making sure he has a solid reentry plan including housing and support on the outside strengthens his file.
The rest is on him.