They shouldn't "pardon". If there were some other form of nullification, that might be ok, but did nothing wrong by modern standards, and "forgiving" him implies that he did, even if that isn't their intent. And any legal relief a pardon would give a living person is irrelevant.
Let the conviction stand as a historical marker of shame.
Instead, they should vote on an apology to him and all the other people wronged by the actions of their predecessors.