1M bitcoins is only around 600 million dollars. He is not even a dollar billionaire, by any yardstick (let alone being one of the richest people in the world). If he decided to cash out that would instantaneously crash the price. So his seeming wealth is ephemeral, for some part at least. On the other hand, what taxes should he pay?
How should they be calculated?
If you look beyond the next 10 years, the bitcoin price could be $100,000 each, that make him much richer.
Wealth is only measured in money, but money itself can be considered as wealth only as long as real wealth can be bought with it. And not some abstract wealth (houses, yachts, girls, boys, whatever), but the wealth that you can make use of. If you had 1 trillion dollars in cash, what could you actually buy with it? You could try to buy Apple shares, for example, but would people who have a controlling stake in this company agree to sell it? When you come to think of it, you inevitably come to a conclusion that being money-rich is not the same as just being rich...
I mean, in the proper sense of the word, i.e. possessing wealth that you are able to swallow and digest, so to speak