I can almost guarantee you he cannot afford a Monaco citizenship. And I still have trouble believing he's got the kind of money to own a 1-bedroom apartment there (would cost close to 1 million euros). If he manages to afford to own a place there just for tax benefits, I am even more curious about his assets, because you need shitloads of money to make use of tax benefits. Hell, my mother was born in Monaco and doesn't have citizenship there.
So my hypothesis is that his family is quite wealthy and he benefited from it. He must be a smart investor too, and he's an awesome skier, but I don't believe all that wealth can come from there.