If he didn't die, he would've been 125 by now, and definitely the world's most famous monarch.
But seriously: suppose he lived through WW2, I doubt it would have much impact on the course of the war itself, though I imagine the Royal Question would have been avoided, as would the murder of Lahaut.
I don't know if he'd stayed in Belgium or would've fled to London. But even regardless of how he would've handled WO2, he couldn't possibly have screwed up his personal relationships with leading politicians as Leopold III managed to do, even before the war. Considering his actions after WO1 (the 'Coup de Loppem'), I also imagine him to be somewhat less tempted with authoritarianism as his son was. He also has a lot more credit, being a 'hero' of the Great War, so I could imagine the popular 'state of grace', which Leopold initially enjoyed when he decided to stay in the country, lasting.
Plus: there would have been no Lilian to play heaven's gift to the antiroyalists. (Well, she would've existed, but her marriage to Leopold would probably have been butterflied away.)
Leopold could still screw it up as crown prince during the war, off course. But he might not be as much a focus of attention as he was as king, or simply be in the position to screw things up as much. Chances are he would've simply succeeded his father, perhaps somewhere in the fifties, and reigned as a perhaps slightly controversial monarch until he died in 1983.
More interestingly, what happens to Belgian politics without Royal Question? Perhaps a more rapid weakening of the Communist Party?
Also, Belgian conspiracy theorists would focus even more on Van Eyck's 'Just Judges', out of necessity.