Having watched far more than my share of prison documentaries, I've always found it very hard to picture the typical ex-con worrying very much about registering to vote and getting to a polling place, or going through the hassle of "absentee balloting" or "voting by mail."
And even those who did so don't strike me as very likely to be voting for bleeding-heart liberal politicians.
I think this sort of thing is more about punishing the "normals."
Don't worry. Democrat activists will do all that for him, including filling out his ballot and casting it.
Look to California's "Ballot Harvesting" to see how this type of vote fraud is accomplished.