It's not him.
While Rex is at the battle of Endor the guy pictured isn't him. The reason Rex is present at all is because he happens to bear a resemblance to old man Rex. It does depict him present in the Galaxy of Adventures short (still not a film appearance) but films take precedent over what happens in the animated shorts and both are wearing different outfits. Nik Sant is still the Rebel trooper present, it's not Rex.