Actually, the original Star Tours is not canon. Rex was brought into canon like Thrawn was, just bringing the character over and creating new stories for them, not everything around them. Star Tours is as canon as the original Thrawn Trilogy. The canon backstory for him in the land is "basically R3X flunked his way out of Star Tours. In the battle of Jakku, in a very epic chase, he crash-landed on Batuu... where he's repaired and turned into a DJ for the cantina." It seems somewhat unclear, but he may have appeared on Star Wars Rebels but there doesn't seem to be anything really linking them apart from an article just stating they're the same without sources.