I might not have a huge amount to pull from, but maybe Metal Gear Solid: The Phantom Pain could be a good choice, I don't think it has too hard of customisation than GTA, and gives quite a bit of freedom.
Maybe a departure if he just enjoys guns play, but I think that Shadow of Mordor or Shadow of War can be kind of fun of feeling like you are going on a rampage after doing some story and harassing some orcs.
Or if he is not indifferent to Batman, there Batman: Arkham City, open world environments which are worth exploring.
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Through dream I travel, at lantern's call
To consume the flames of a kingdom's fall